Does any one know a good simile or metaphor?

"Life is like a box of chocolates." from Forrest Gump (simile). After that it says something about "you never know which one you're going to get," so you could maybe incorporate that into your letter by saying "Life (or School) is like a box of chocolates. You never know which one (teacher) you're going to get. Luckily, I got a good one" or something to that effect...