What you need:
Two similar balloons
Two sports drinks bottles
Something to cut the ends off the bottles
Cut the lock and screw ends off both bottles, keep the ends unlocked and stretch a balloon over each one.
Blow up one balloon a little and lock the cap. Blow up the other a lot more and lock.
Tape the caps together and unlock – air flows from the small balloon to the larger one and inflates it even more!
Results & explanation
The small balloon’s surface is very curved, so most of the tension in the rubber squeezes down on the air, increasing its pressure. The big balloon has a flatter surface, so the tension in the rubber mostly pulls on other rubber.
PS – Marvin & Milo are © IOP and can be freely used in schools. Please do get in touch if you’d like to modify or use them for commercial purposes.