How many lights does the garbage truck have? Why?

Are you laughing? At that time, Xiaohua imitated the music of the garbage truck (just like the sprinkler playing music, the garbage truck in Taiwan Province Province may also play music while driving). It makes the sound of "Deng Deng Deng", so it has nine lights.

This is a children's song. The lyrics of Taiwan Province Province are: "There are several lights in the garbage truck, staring at each other. . .