The Peeing Boy Experiment (And the Science Behind Why He “Stops”).

If you want to watch your kid’s jaw hit the floor, this is the experiment. We call him the peeing boy and yes, my house has not stopped giggling about him since we made ours.

Here’s the trick: you fill a bottle with water, poke a straw through a hole near the bottom, screw the cap on tight… and nothing happens. Ideally, not a drop.Then you loosen the cap, and suddenly he pees. Every time. My kids asked me to do it about 20 times in a row.

(If you let your kids do it, it might come some water to leak).

How It Works

I’ll be honest, my first thought was “there’s no way this is just air.” But it is.

When the cap is screwed on tight, the bottle is basically sealed. For water to leave through the straw, air has to rush in somewhere to take its place and with the cap tight, there’s nowhere for that air to get in. So the air pressure inside the bottle (plus the water) pushes back just enough to hold everything in place. It’s a standing stalemate.

The second you crack the cap open, air can sneak in from the top. That breaks the stalemate, gravity wins, and the water finally has somewhere to go straight out the straw. Boy pees. Kids scream with laughter. Repeat forever.

It’s a nice, hands-on way to show kids that air isn’t “nothing” it actually pushes on things, all the time, even when we can’t see it.

Grab the Free Printable

We turned this into an actual printable boy so you don’t have to draw one freehand (my drawing skills did not do him justice). Download the free printable The Peeing Boy, tape or glue him onto your bottle, and let the science begin.

Try it with your kids and tell us in the comments: how do you think he stops? We’ll break down the full science in our next video.

STEM for kids,free printable ,easy science experiments, water pressure. Interested in more? Check out our science experiments for kids.