The Science Behind: Sandcastle Building

With Labor Day – and the last gasp of summer – upon us, a few of you lucky souls may find yourselves on a beach this weekend, and with that the chance for some sandcastle building. I’m here to help you leverage that opportunity to wow your children with your skills and knowledge.

We all probably have memories of building sandcastles as kids, whether it was at a lake, a beach or just in a sandbox. I know for me, it went something like this: fill up a plastic cup with wet sand, let it sit for a minute, then flip the cup quickly onto the beach and then as carefully as I could, remove the cup. And usually, watch my “castle” crumble into more like a mound.

But my mind was opened to a new world of possibilities when, at a family reunion I attended earlier this summer, one of my relatives found a professional sandcastle builder to do a session with us. (I know – WHO KNEW there was such a thing??)

Janel, our Sandcastle Instructor, explained that the key to a successful – and sturdy – sandcastle is all in the SCIENCE!

The science behind sandcastle building has to do with the interaction between water and sand, and with a special force called “surface tension.” (Yes, pull that word out to impress your children.) In case you need a refresher from your high school physics class, surface tension is the force that causes water molecules to be attracted to one another. Think about what happens when you fill a glass of water to the VERY VERY TOP – you can see a domed effect where the water molecules are holding together. That’s surface tension in action.

Surface tension is what gives wet sand its strength for great sandcastle building. The water molecules coat the grains of sand and hold them together, forming water-drop bridges in the gaps between the grains. Too little water, and surface tension can’t hold the sand grains together. Too much water, and the water turns into a lubricant and your sand collapses and flows with the water, making a mushy mess. 

We learned that the magic ratio of sand to water is about 8:1 – that is, 8 parts sand to 1 part water. The other key is making sure EVERY GRAIN of sand is coated with water to maximize the number and strength of those water-drop bridges holding the sand together. (See tips on how to do that in our Technique Tips below.)

Now that you know the science behind surface tension and sandcastles, here are some tips to build the best ones!

You don’t need any fancy equipment to make a fancy sandcastle. Everything you need can likely be found in your kitchen, garage or local hardware store.

  • A long-handled, lightweight shovel: a long-handled shovel with a small scoop. The sand-sculpture task that feels the most like work is filling up the bucket with sand! A little garden spade is better than nothing.
  • A bucket or two: we used two 5-gallon buckets – one with the bottom cut off for forming the main castle (which made it much easier to mix the water and sand in, and then just lift it off) and one filled with water to add to sand for all the additions to our main structure. Of course, buckets of any size will do! Smaller buckets won’t be as heavy, so don’t worry about cutting the bottoms off.
  • Plastic molds: Use plastic food containers to make towers and building blocks. 
  • Carving tools Look in your kitchen for smoothing and shaping tools: for basic shaping, plastic knives, paint scrapers and trowels. A pastry knife or small spatula work well for detail work. Melon ballers (finally! A use for that!) or just a tablespoon work great for scooping out round windows.
  • Finishing tools: A soft-bristled paintbrush is handy for smoothing surfaces. A paper straw works well for blowing loose sand out of detailed carving.

These are tips we learned for building a sturdy base for your sandcastle.

  • Fill your building bucket with sand: Again, we used a bottomless 5 gallon bucket for this, but any size will do. To get the right ratio, fill your bucket about 90% full with sand.
  • Mix up your sand: Pour water into the building bucket where you put your sand – and fill it right up to the top. Then reach in with your hands and really mix and mix and mix to get all the sand particles in the entire bucket all coated with water. Depending on the size of your bucket, this might be a whole-arm activity. (Fun fact: this is what cement mixers do!)
  • Compact wet sand:  Packing sand strengthens and tightens the little water bridges that hold the grains together. Tap the sides of your bucket with your fists or your feet until the sand on top looks compact and almost dry. Then carefully lift off your bucket – ta da! You’ve got a clean slate sandcastle base!
  • Carve, build, decorate: Use your finishing tools – and your imagination! – to create the sandcastle structure of your dreams.

Once you have a sturdy base, you can make your own sand-dragons and sand-boats, and create beachside castles with watery moats. Use cups and old pots and pans and cookie cutters and whatever else you can find to mold interesting shapes. Keep a bucket of water close by to keep your building material sticky. Just watch out for sneaky waves (or little brothers) that can bring the whole thing tumbling down.

And though summer may be officially ending this weekend, you can bust out your sandcastle science facts to impress kids in the sandbox anytime!

Introducing Our Newest Arrival: Panda Crate!

We are thrilled to announce the launch of Panda Crateour newest line for babies and toddlers

Panda Crate was developed especially for little ones (aged 0–24 months), grounded in the science of early childhood development, and made helpful, practical, and convenient for parents. We worked closely with researchers at Seattle Children’s Hospital to ensure that all the products and information delivered in each crate are perfectly suited for your child’s developing brain

The products and information included in each Panda Crate are developed and tested to be developmentally appropriate for specific age ranges, so you’ll always get just the right crate at the right time. In fact, the Panda Crate line is broken up into five separate age bands:

    • Birth-2 months
    • 3-6 months
    • 7-12 months
    • 13-18 months
    • 19-24 months

We know first hand that the first months (and years) of your child’s life can be often busy, sometimes confusing, and seemingly never-endingly sleep deprived. We’re here to help parents feel more confident in answering all those questions around “What’s best for my baby? Is this normal? What should I be doing with her now??” And we will always do it in a way that is easy (we distill all the research for you), convenient (all the products and info are delivered to your door) and fun (because, goodness knows, parenting is already enough work)! 

The bottom line: Panda Crate allows you to focus on bonding with your baby while we take the guesswork out of what’s research backed and developmentally appropriate

If you’re familiar with our other subscription lines, you will notice one difference with Panda Crate: these deliveries will arrive at your home every other month instead of monthly. The contents of the crate, however, will be roughly double that in a monthly crate. We’ve made this change to allow us to group together more toys and materials for parents to use across a two-month stage — to better support the rapid pace of babies’ and toddlers’ development and the wide spectrum of timing along which individual children grow.

With the launch of Panda Crate, KiwiCo now has the perfect solution for any kid — or kid at heart — ages 0 to 104! Check out Panda Crate and all the rest of our lines at

Cheers from our family to yours,

Sandra – CEO, Founder, Mommy of 3

Science Fair Projects for 9th Grade

These science fair projects for 9th grade are perfect for older kids to explore advanced science concepts and tinker with some more exciting materials.

1. Pressure Bottle Rocket

Water squirters work by pressurizing reservoirs of water with air. As you pump the squirter, it gradually adds air to the reservoir, increasing the pressure. When you pull the trigger, the pressure is allowed to release and a stream of water shoots out!

The pressure bottle rocket works thanks to Newton’s third law of motion, which states that for every action, there is a reaction. As you pump up your rocket with air, pressure builds up inside. Eventually, this pressure pushes the rocket off of the cork and bicycle pump, and expels the water in the bottle downward — this is the action. The reaction to this downward movement of the water is the rocket’s own upwards movement, sending it high into the sky.

Go Beyond the Science Fair

KiwiCo offers awesome chemistry lab kits. Learn more about our Glow Lab, Fire Lab and Vortex Lab.

Check it Out »

2. Flying Teabag

Make a tea bag fly with heat! The flying tea bag experiment is a similar concept to a hot air balloon, but you can do it right at home.

Did your tea bag lift into the air? When the tea bag was lit, the flames started to heat the air within the teabag. The heated air rose above the cooler dense air (similar to a hot air balloon!). Once the teabag was burned, we were left with the lightweight ashes of the tea bag. The lightness of the ashes combined with the heated air caused the tea bag cylinder to lift into the air.

3. Flaming Torch

Create a whirling fire torch of your own. The flames spin upwards into a vortex from the rising heat and wind. Follow along to safely create a whirling fire torch yourself!

Stand back and watch your fire form a helix. Experiment with the spinning speeds to see how it affects the shape of the flaming tornado! The flame will last a few seconds and eventually die out as the paper towel burns. Exercise all caution while the flames are live. The beaker may be very hot so use tongs to keep your fingers safe!

4. Electromagnet

Give this experiment a try, and see how many paper clips you can pick up with your electromagnet!

Unlike the magnets that are used on refrigerators, electromagnets are magnets can be turned on and off depending on the flow of electricity. The electricity that flows through the wire allows the molecules on the in the nail to attract to certain things.

5. Rubber Band Racer

Using simple household materials and tools, it’s super easy to create a rubber band powered racer and experiment with wheels, rubber bands, and different surfaces to see how fast your racers can go!

To wind up your car, carefully turn the large wheels of the car so that the rubber band winds around the back axle. Let go and watch your car spin off!

Continue experimenting with your car and try different size and length rubber bands. What combination works the best?

For more amazing science fair projects, be sure to check out:

Explore 14 Different Experiments with KiwiCo’s Chemistry Sets

Kids are fascinated with chemical reactions. But learning about combustion, bioluminescence and other reactions takes a lot of work. You need both a knowledge of chemistry and the supplies to conduct experiments.

That’s where KiwiCo’s chemistry sets can help. Our chemistry sets come with all of the supplies that you need. They also provide easy-to-understand context for young scientists, so kids can learn about scientific concepts. Our chemistry set bundle includes:

  • Safety equipment, including safety goggles and gloves
  • Easy-to-follow instructions that also explain key scientific concepts. All of our chemistry sets help kids learn more about polymers, plastic, solids and liquids.
  • All the chemicals and supplies you need to conduct the experiments
  • The parts you need to create a burner, mixer, UV lamp and spiral stand

With our Glow Lab, Vortex Lab and Fire Lab, we provide everything you need to explore the wonders of chemistry.

Fire Lab Activities:

The Fire Lab helps kids learn about combustion, oxidation capillary action and other concepts. Here’s a list of activities and experiments that come with the Fire Lab:

  • Learn about Combustion Chemistry
  • Fire & Fuels Experiment
  • Fire & Oxygen Experiment
  • Flame Vacuum Experiment
  • Solid Flame Test Experiment
  • Liquid Flame Test Experiment
  • Burning Steel Experiment
  • Color Burner Experiment

You can get the Fire Lab here. 

Glow Lab Activities:

Things that glow in the dark are awesome. Understanding why things glow it the dark is awesome too! The Glow Lab helps kids understand concept such as fluorescence, phosphorescence and chemiluminescence. Here’s a list of activities and experiments that come with the Glow Lab:

  • Learn about Cold Light
  • Learn about Invisible Light
  • Fluorescein Experiment
  • Zinc Sulfide Experiment
  • Luminol Reaction Experiment

You can get the Glow Lab here.

Vortex Lab Activities:

High-speed mixing has never been so fun. The Vortex Lab helps kids learn about the science of solubility, diffusion and oxidation-reduction. Here’s a list of activities that come with the set:

  • Build the Mixing Machine
  • Motion & Kinetics Experiment
  • Heat & Kinetics Experiment
  • Iodine Clock Reaction
  • Blue Bottle Reaction
  • DIY Oil & Water Experiment

You can get the set here.

Interested in all three sets? Check out our Chemistry (3-Pack) to save on the Vortex Lab, Fire Lab and Glow Lab. Get it here.