While their primary purpose is of course to keep your hair out of your face, relatively dry or protect from chlorinated water, or to reduce drag if you are swimming competitively, if you have a spare latex cap; (or your want to buy one from our swimshop) try this out!
If you, and some friends fancy testing it, it's a lot of fun, but use caution- stretching a cap to it's maximum capacity leads it to split and this can be painful if it catches you as it snaps back.
The World Record for this is held by 'Team Badekappe (Germany)' who managed to have 4 swimmers fit inside a standard latex cap, on 19 July 2015.
For this record a normal size swimming cap was used, it was stretched underwater by a team of six and then 4 participants entered the cap sitting into it and pulling the rim of the cap to their waists without breaking it.