Do dicks actually "twitch" occasionally?

Dumbass question but hey, I'm curious.

Basically I'm a senior and the times I have sat over a guy's lap for any reason I'd occasionally feel something smaller than his leg twitch, and if it's not a body part or small muscle then I guess it's the chestburster from Alien or something. And honestly, if a guy can ask on here if girls defecate then this question isn't that stupid.
Do dicks actually "twitch" occasionally?
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