system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the penis has no muscles why can you sometimes stiffen it more at will or make it twitch?
gpt
Your pelvic floor muscles are located at the base of the penis and control things like sphincters. Because the penile tissues are all attached around this region, flexing those pelvic floor muscles will indirectly pull on your penis and it will appear to twitch.