Pretty hard,I cant get it to hurt though.I have an innie I'd imagine it would hurt If you had an outie
I once stuck my finger in my belly button and pulled out a ball of belly button lint. It was grey and smelt funny.
If you poke anything "pretty hard" I'd imagine it would hurt. Haha XD Mine doesn't hurt when I poke it...0.o Maybe you're preggers, dude (somehow...).
Mine hurts sometimes when I'm playing with it (lol). I've always assumed it has something to do with the fact that it's scar tissue. I get the same kind of pain on some of my scarred areas on my body (really sharp stabbing pain under the skin)