I can't whistle.
I've always tried - ever since I was little - but never been able to get anything more than a breathy, barely audible squeak out. I'm convinced it's a result of years of playing the flute and piccolo...the embouchure, the muscles around the mouth, is totally different to play a flute than to whistle. While it looks like to whistle, one needs to pucker up the lips and focus the sound straight out, flutists tighten the corners and soften the bottom lip, controlling the air stream out and over a flute's mouthpiece.
You'd think, after five-plus years without touching a flute or picc, I'd be able to change things and get a whistle out, but I guess old habits die hard. After the sheer amount of flute-and-picc playing I did as a high school and college student, it must just be too ingrained in me...oh well!