Did you ever slip into a warm bath and find it so relaxing that you just wanted to close your eyes and fall asleep? Yea, well, sperm feel the same way.
When sperm are placed in a warm bath, they have a hard time keeping their little eyes open (sperm don't really have eyes) and they fall asleep and don't move.
A German woman who is an industrial design graduate from the University of Munich won an award for creating a warm bath for testicles that puts sperm to sleep and acts as a male birth control device.
She was diagnosed with cervical cancer from using birth control pills. Her and her partner decided to find a birth control method that he could use and - GUESS WHAT? - they couldn't find one they liked. So, she invented one.
It's called the COSO male birth control device. It's a little bath tub that you fill with water, hit a button to heat the water, then dip your balls into the warm bath for a few minutes. Sounds fun.
I'm not sure how long the sperm stay asleep, but while they're asleep they're not swimming around looking for an egg.
Here is the link to a Vimeo video showing how it works:
*Spoiler alert! No live balls were used to make this video.