50 minutes into this video the professor puts a balloon into liquid nitrogen that's about 1/4 of the room temperature, but its volume decreases to much less than 1/4 of its original volume. Why? Right after the balloon demo, he talks about tennis balls. Why if you open up a can of tennis balls, don't play with them for a few days, do they become useless? And how does this relate to whether the pressure in the sealed can of tennis balls was less than or greater that 1 atm?