What do you call a snail on a ship?
My brother says that he first heard this from one of his friends. The punchline makes use of a pun on the words snail and sailor, combining them into one word: snailor. Derek likes this joke because snailor is a funny word! When asked if he knows any jokes, this is typically the one that comes to his mind first.
As a current first-grader, children at that age are becoming more creative with language and are learning how to express themselves more clearly through written word and oral speech. At this time, they are also figuring out what words can and cannot go together, in addition to distinguishing nonsense words. Combining two words to form snailor makes no sense outside the context of the joke, but the combination of snail and snailor is easily seen when the punchline is accompanied by the question. First-graders know that snailor is not a real word, but the fact that it exists in this joke makes sense for them: they know what a snail is, and they know what a sailor is. The jump from the two separate words to one word is easily made, since the first syllable of sailor is nearly identical to snail. Since the two words are relatively simple, it is easy for a 6- or 7-year-old to see the cleverness of the term snailor, and thus find the joke amusing.
Another interesting feature of this joke is its alliteration. With snail, ship, and snailor all beginning with the letter s, the joke is probably easier for children to remember, and it also has a nice ring to it. Additionally, all of the words (except snailor) are only one syllable, possibly contributing to the being more easily remembered among first-graders. The joke is also fairly shorta quick question, a quick answer. Since childrens attention spans are still growing at this age, a short joke fares better on the playground than does a longer one.