Why are tennis balls sold in hermetically sealed cans?

[DSYoungEsq]

@CookingWithGas

I mean “pop off” like a champagne cork.
I mean “pop off” like the lid of a sealed Tupperware container in the microwave.
I mean “pop off” like the top of a botulism-infected can of Campbell’s.

I would at least expect a big “whoosh” of air coming out.

There IS a whoosh of air coming out. It’s just not as big a whoosh as perhaps you are thinking, given that the pressures aren’t so great. More like a “pssssst” really.