I start with the letter e, I end with the letter e. I contain only one letter, Yet I am not the letter e! What am I?

Why this works

This riddle plays with the different meanings of the words involved. It starts and ends with the letter "e," which refers to the first and last letters of the word "envelope." The phrase "I contain only one letter" is a clever twist because it hints at the fact that an envelope often holds a single piece of mail, or letter, rather than referring to the letter "e" itself. Thus, the answer is both a play on the structure of the word and the function of the object!