My parents showed me this when I was a kid; I think they'd known it from childhood:
They had to explain to me how to read it. "YY" is "two Ys," or "too wise"; the whole thing is thus "Too wise you are, too wise you be; I see you are too wise for me."
I don't know whether that tidbit would count as a rebus, but it certainly lives in the same general part of the wordplay universe. A rebus is a phrase or sentence written as a combination of words, small pictures, and individual uppercase letters; the names of the objects denoted by the pictures are usually homonyms of the intended sounds or words. ("Rebus" is from Latin meaning "by things.") For instance,
would be read as "I can see you." Of course, not everyone has the same ideas about what a given picture represents; even with an icon whose conceptual meaning is universally understood, it may not be obvious what specific word is meant. The above could be misread, for instance, as "Look, wastebin, see you." Misinterpretation (intentional or not) can lead to humor; misreading unclear rebuses can be more entertaining than finding the originally intended answer.
Rebuses also employ a sort of letter arithmetic. For instance, "[picture of can] - C" indicates "an"; "[picture of telephone] - T - 1 + [picture of ant]" indicates "elephant." And so on. Unfortunately, most rebuses aren't very interesting; they tend to be very simplistic and aimed at children (or else stupid and printed on the inner surface of bottle caps).
Another form of wordplay that's often thrown together with rebuses but is often more interesting involves using descriptions of attributes of words as parts of the words. For instance,
can be read as "I'll oversee your undertaking." Colored words can be used too, for variety:
There was a group who used to post pages of such puzzles on rec.puzzles a few years back; if anyone can provide me with contact information for them, assuming they're still around, I'd be happy to post that information here.
Yet another related item is the practice of treating numbers (and homonyms of numbers) contained within words as multipliers:
It was a dordor house, but still afdadadadable. It had a derful kitchen, with plenty of space for chopping potoooooooo and making nana salad. It even had a sunny ririririririririum where Madame Nana-nana-nana-nana could practice her Tarot readings all day, and Professor Sysysysythe could examine his atdandandandandandandandandandance sheets and dream up new ways to make his students pay attiontiontiontiontiontiontiontiontiontion. The Professor saw the house on the way to a umumumum one day and fell in love with it. But the previous owner, being a thrightthrightthrightthright fellow, felt obliged to tell them that the place was haunted... (answers)
And so on. All of this reminds me of William Steig's CDB, as mentioned in a previous column, filled with phrases like "MNX" (ham and eggs) and "LE S N D LF8R" (Ellie is in the elevator); if you haven't seen this book, go find a copy.
Vanity license plates often use similar methods to get their messages across, from CDB-style letter-naming (as on the much-commented-on car in the Bay Area with license plate "IML8" (guess what color and make the car was)), to mirror writing (3M TA3), to upside down writing (PV3HPV3P). The most impressive license-plate-word project I've seen is the photograph of a couple dozen vanity plates from around the US that spell out a semi-phonetic version of the Preamble to the US Constitution (the first plate has the letters "WEE" on it, I believe, followed by a "THUH" plate, and so on; many words cross plate boundaries). I don't know for sure that that piece uses real license plates, but I hope it does.
My "most obscure license plate" award (that is, most obscure that I could figure out; there are plenty that I can't decode at all) goes to the Silicon Valley plates reading "0X2A"; in computerese, a "0x" prefix implies hexadecimal (base 16 numbers), and "2A" in hex is equivalent to 32+10=42 decimal. Presumably the owner of the vehicle was a Douglas Adams fan as well as being a programmer.
The best license plate I've seen in recent years is the one that reads ZPED2DA. I spent some time puzzling over it—"zee ped today"? What could that mean?—before it finally came to me: Z PE D 2 DA is about the only way one could squeeze the phrase "zippety-doo-dah" into the seven or eight characters allowed on a California license plate.