Come on, everyone has one...a simple thing that you've tried to learn, but just can't. Mine is that two-fingered piercing whistle. My brother can do one that could split your eardrum, and he tried and tried to teach me, but I just can't get it.
Or maybe it's something that everyone can do but you just never had the chance to learn, like one person I know who can't ride a bike.
Fess up, people!