What does the joke told by the little flower in the laughing flower mean? Why are there nine lights in the garbage truck?

There will be music in some parts of the garbage truck. The music of the garbage truck there is the first sentence of For Alice. Because the piano music has no lyrics, Xiaohua sings Deng in her own way. "Light" and "light" are homophonic, so I asked how many lights there are, that is, how many pedals there are. That music is nine notes, so it's nine pedals, so it's nine lights.