Blow different volumes of air into two balloons connected by a T-tube. When the valve connecting both balloons is opened, air will flow from the smaller balloon to the larger balloon.
Beware, when doing this demo with rubber balloons, you must be careful to blow a large enough difference of air into each balloon, else they are susceptible to reach another equilibrium where the balloons have unequal radii. For an example of how *not* to do this demo, see the following:
For a discussion of why exactly this can happen, cf. AJP volume 46, pg. 978
Location: C3-3