She choked on water when she saw it.

I was at my gf's house and her family was gone for the next two days. Needless to say a lot of hanky-panky happened. After one of our "sessions" we hit the pool. I was wearing a loose blue pair of swim trunks and she was wearing a smoking hot black bikini. We were mostly just talking instead of swimming but eventually I got hungry and went to make a sandwich. Getting out of the pool my trunks were hanging a bit and she made a joke about seeing my butt. I said "If you want to see it so bad then you can." and dropped my swim trunks. (She had a wooden fence so no neighbors could see us) She starts dying laughing and I'm going along with the joke flexing and shaking my butt. Eventually the joke ends and she starts swimming with her mouth in the water. I turn around and due to this being both post coitus and just getting out of the water my penis has gone beyond shrinking and genuinely looked like a button. She goes to laugh and just inhales a bunch of water and starts coughing it up. I obviously hop in to see if she's ok and help her out of the pool. "Are you ok?" I asked. "Yeah I'm fine I just started laughing and inhaled a bunch of water." After she said this she very obviously glanced back down to my button penis and started giggling again.