(Kelly of the Clue Crew gives the clue from a lab.)  Stretch a balloon over a glass, secure with rubber bands, then place a straw on top & you have a simple version of this device that measures air pressure
Gather rubber bands to keep the balloon tight on the jar, and a 
lightweight ... and stretch the remaining part of the balloon over the mouth of the 
glass... [DOC] [TLE] Balloons In Science [PAR] TRY TO HAVE AS MANY ON HAND AS YOU CAN TO DEMONSTRATE ... Use 
the rubber-band to keep the balloon in place over the bottom of the top half of the 
bottle.