鮎は魚だよ！ The ayu is a small Japanese river fish.

According to "Looking for the Lost," the following joke is well known among Japanese students of English:

One student of English says to another student of English, "Are you a fish?" The second student either replies, "No, I am not a fish," or some variant of this, or looks at his interlocutor as though he were a fruitcake. Whereupon, the first student says, "No, no, ayu" (are you) "wa" (a) - subject particle - "sakana da yo" (is certainly a fish), and the second student's face clouds over with embarassment or lights up with mirth as he realizes that he has been treated to a pun. That is a fairly typical example of Japanese humor...

That's right, Japanese people love puns. The pun, hardly even considered funny by Western standards and often met with groaning sounds and the rolling of eyes, gets quite a different response in Japan. Last year I mentioned a pun written by one of my students. This love of the pun, known as share 洒落 in Japanese, most likely stems from the limited variety of sounds found in the Japanese language. This limited variety results in a multitude of words that sound similar as they are constructed from the same syllables, just in a different order, making it quite difficult to memorize vocabulary (and if you're slightly dyslexic, you might as well give up entirely). More importantly however, at least in terms of pun creation, are the multitude of words that sound nearly identical, aside from intonation (doonigigo 同音異義語 or homonym), but have entirely different meanings.

Here's another example, in that sense:

A: 鎌倉の大仏はいつたったか知っている？
B: たしか、13世紀だったかな。
C: 違うよ！大仏はずっと座っているよ。

Described in "Looking for the Lost":

One man says to another man, "When was the Great Buddha of Kamakura erected?" (The Great Buddha of Kamakura is an 11.4 - meter - high bronze statue of Amida seated in meditation). The second man replies something along the lines of, "Oh, round about the middle of the thirteenth century, wasn't it?" Whereupon the first man says, "No, no, it is still sitting down." This joke, also a pun (the staple of Japanese verbal humor), depends on the fact that the verb tatsu can mean either "to put up" [建つ] or "to stand up" [立つ]. Get it?