A Snowball is a poem in which each line consists of a single word, and each successive line contains a word exactly one letter longer than the word above it. You may make a poem of as many lines as you wish, provided each one-word line succeeds the previous line by a single letter.
Here is an example from one of last year’s winners, Maryann Younger:
Send your Snowball to Puzzler by Sunday noon, Pacific daylight time. You may enter as many Snowballs as you wish.