Stop me if you've heard this one. Two men aged around 50 enter a sushi restaurant. One orders a raincoat, the other a garage. What looks like the beginning of a "Monty Python" sketch is in fact the stuff of a most typical oyaji gyagu (おやじギャグ), or old man's joke/gag. Such jokes normally center around words with similar or identical reading, but with an entirely different meaning. Linguists call these homophones.
In the above case, the joke derives from an intended misreading of the words kappa and shako. While kappa in the context of a sushi restaurant is usually interpreted as an abbreviated form of kappa maki (かっぱ巻き, a cucumber sushi roll), kappa when spelled differently (合羽) means raincoat. This is also available as an English loanword, レインコート, which is what one of the guys used when making his order. As for the other one, his joke was based on the fact that shako (蝦蛄) is not only a type of shrimp, but also a dry place to leave your kuruma (car) (車庫), which in Japanese is equally known by the katakana word ガレージ, garage.
One common feature of the oyaji gyagu is that those homophones frequently occur in a single, more or less meaningful, phrase. Here are a few well-known examples of this type: