The joke is that they know it is an egg. The user, somewhat like myself, is clearly trans, and “egg” means something important to the trans community. The extra question marks and the “:3” set the rest of the humorous/teasing tone. The post on here is itself the same kind of subtle meme, with the post text just saying, quite plainly: egg.

I imagine this wouldn’t be intuitive or amusing unless you’re really in the loop.