One balloon inflates another

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(C)Properties of Heat and Matter - C+27: Surface Tension and Capillary Action

Two identical balloons are connected by a valve that can be closed. One is more inflated than the other. When the valve is opened the less inflated balloon will deflate and the more inflated balloon will get slightly larger.
UCB Index: 
C+27+35
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Revolving 'drumhead' with manometer shows pressure in water tank.