one third full of water. put a piece of cardboard over the top and place your palm over the cardboard. then tip the glass upside down!
result: no water escapes because the water inside the cup is lighter than the air around the glass.
fill a 2-liter pop bottle half full of water. put a long straw into the mouth of the bottle and make a seal of tape over the bottle top and sticking to the straw. now blow into the straw to increase the air pressure inside the pop bottle.
result: the air pressure inside the bottle will push the water out of the straw.
name two ways of speaking about air 