To me, aged about 20 minutes younger than I am now:
Dear trousers (yes I still use the lower-case, even in the future),
That new knife you bought for chopping up food. It's very, very sharp. That's why you bought it.
So if you happen to be surprised when you cut your fingers open when you're preparing your dinner - because you've forgotten that the knife is very, very sharp, even though that's why you bought it - then don't expect any sympathy from your future self. In fact, your future self thinks that you're a bit of a twat right now.
Yours, trousers (with sticking plasters on his fingers).
ps you're aging well.