Bathtub STEM? Sure, why not? You will just need a few things to have more than a few moments of fun with a perfect balloon powered boat.
For step-by-step video instructions for kids to work independently, head to The Winkle YouTube channel!
Collect the following items for a balloon boat:

1 flat kitchen sponge.
1 balloon 9”-12”
5/8” diameter plastic tube. 3” will do. (Find this at the hardware store.)
1 zip tie
How you do it:
Cut a point into the boat.


This next step will secure the plastic tube/balloon to the sponge to ensure the tube points backwards. Your boat may work without this step, but I would recommend not skipping it. You can also do this step after you thread the balloon and tube through the sponge.



Attach the top end of the plastic tube to the balloon.

Find the center and poke another hole for the balloon to go through.



Hang on to the balloon nozzle and blow through the tube until the balloon is full.

After you get the balloon blown up, put your finger over the hole until you get it in the water.
Place the boat in the water and let it go.
Tips:
The balloon may lose its power after a while so be sure to have some extra balloons on hand.
While you are at the store, check out all the other cool options from The Winkle Crate