OK, I'll bite. What song?
Okay, I took forever to write this and in the meantime the question was thoroughly answered. My answer contains an anti-schoolhouse rock rant, though, so I'm going to post it anyway:
I don't remember what it's called, but it's from Schoolhouse Rock and it's a song about the number three and how you can multiply it by all numbers from one to ten. I think the chorus goes "Three... it's a magic number," which apparently is the slogan for the number three. Then all the cartoon children sing a times table enthusiastically. It's horribly catchy, and horribly horrible.
I really dislike schoolhouse rock, for aesthetic reasons and also because I think it's a really gimmicky way of tricking kids into learning. They could have made a kids' show which illustrated concepts in an accessable way, or demonstrated the value of the material by showing people using it, but instead they basically took a pretty dry list of facts and times tables and set it to music. A lot of the material we want kids to learn has the potential to be genuinely valuable and/or enjoyable, but Schoolhouse Rock just places a standard boring curriculum in the same room as singing cartoons, in the hopes that kids will conflate the two. It's like they're trying to send the message, "LEARNING CAN BE TOLERABLE!!! (if used in conjunction with tolerable things, like television.)"
The other bad things about Schoolhouse rock are:
1) Somtimes people perform it as a play, and other people have friends involved in its production and so are obligated to see it. In my days as a theater person I was dragged to such performances on multiple occasions.
2) It has a song about how great manifest destiny is.
I do like the Preamble to the Constitution song, though.