It's 3 a.m. and once again my sleep has been interrupted by a pun.

This time, I was dreaming that a mollusk had just been asked if it needed an STD screening, and I woke up as it responded, "Nope, I'm shell-ibate."

Although I don't recall ever hearing this pun in waking life, I imagine versions of it do exist. Reverse engineering from my dream, here's an easy-to-tell version:

What did the mollusk say when asked if it needed an STD screening? "Nope, I'm shell-ibate."