system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a man ejaculates, what creates the pressure for the semen to fly out?

Are there muscles in the balls or something?
gpt
Testicles only produce the sperm that makes up about 5% of the volume of ejaculate. The rest is fluid produced by the prostate and other internal glands. It is ejected by forceful contractions of the muscles at the base of the penis.