My sister has the worst memory ever, she doesn't remember anything. So I tell her about stuff we did when she was a kid, things like the fact she was the clumsiest kid ever. She somehow managed to fall over her own feet while stationery and split open the top of her forehead, which my mum sorted out (one of the many benefits of having a nurse for a mother) with some butterfly strips. I know it doesn't sound funny typed out like this, but stories like it have my sister in stitches. I also tell her about other stupid things that happened, the time she ate a whole clementine, peel and all, or an entire apple and when my mum asked for the core, my sister didn't know what that was. She was a weird kid, prone to doing and saying things no one else would, often in her own little world. It was OK, they knew her there.
The funniest thing about telling her stories of herself is that, she laughs so much that water will come out of her nose if she's just had a drink. She gets the giggles so much that on occasion she's started to choke a little. My mum isn't too impressed when that happens, although she often joins in the story telling. I've never met anyone that finds stories of their own mishaps so funny, but then I suppose if you can't remember them happening and they're told in a humorous manner then it would be hysterical.