While I was considering this water displacement experiment as a means of demonstrating, in a very basic way, the density and mass of different objects and the impact that can have, my oldest daughter suggested to me that “every kid already knows that bigger things take up more space in the world.”

She’s correct, probably. Most kids, even young ones, likely do understand that every single thing in the world takes up a certain amount of space — even a speck of dust! — and that, for example, their parents take up more space than they do. Ask even a preschooler if one could, generally speaking, squeeze more little kids or grown-ups into an elevator and they will certainly provide the correct answer. But unless they’ve actually tried to perform that human elevator trick their understanding of density, mass and space is only theoretical. This water displacement experiment will make it very real and very fun, because who doesn’t like getting a little wet?

### What You’ll Need

• A clear measuring cup or glass pinch bowl
• A plate
• Water
• Coins or LEGO bricks (LEGO do not sink but are still fun and colorful to use)
• Paper
• Pencil
• Kitchen towel

### How to Set Up This Water Displacement Activity

1. Fill up the cup to the brim with water.
2. Place the full cup gently onto the plate.
3. Organize your LEGO bricks by size or coins by denomination (bonus learning activity for little ones: have them determine which bricks/coins are bigger and which are smaller, and line them up in the proper sequence)
4. Make a chart on paper of the different size bricks or denominations of coins you’ll be using for this water displacement experiment.

### How To Do The Water Displacement Experiment

Start by gently dropping dimes (they are the lightest U.S. coins) sideways, or the smallest LEGO bricks you’ve collected, into the cup of water from a very low height (no plopping them in cannonball style!) one at a time. As each brick or coin gets added to the water, the water will naturally rise as space inside the cup is becoming occupied by the LEGO or coins — this is water displacement! The water must make room for the new objects being added and so the water will rise and eventually spill over. In case you were wondering why a plate, it’s for this moment!

There should be a nervous energy building with each added brick or coin, the same kind of ‘edge of the seat action’ you experience while playing a game like Jenga because you never know which coin or brick will be the one to force the water out onto the plate. In fact, feel free to make this experiment THAT kind of game by taking turns adding the coins or bricks! Once the water does spill over the edge, write down the number of small LEGO bricks or dimes it took to force that action.

Dump and refill the water, then repeat these steps with all the bricks and coins. By the end your child will have the scientific proof of what my oldest daughter thinks they already know in theory, that the bigger and heavier the item, the more space it will occupy, and the faster it will displace the water up and out of the cup!

They will also have scientific proof that a lot of coins and bricks need to be dried with a kitchen towel!