Balloons

blow two balloons, then open the tap

blow up two balloons on one tube, open the tap between them, and watch which way the air goes

A rubber balloon pushes back hardest when it is still small: its pressure climbs to a peak just after the slack is taken up, then falls as it grows. Join a small balloon to a big one and the air runs from the small into the big until the pressures match. Join two barely blown ones, below the peak, and they even out.

Concepts: laplace-pressure, elastic-instability, elasticity, pressure-volume-curve, non-monotonic-response.

An interactive you play with one thumb, on Wundr.