These stickman-looking creatures you find in the Mouthsinging clips are simple creatures. They have one mouth and they sing. Thats all they do. So thats why their songs are simple text-songs. The chosen songs is just for fun. Greater songs often have long intros.
I learned one day; if you synch the sound to STREAM, you can easily animate lip-singing. So I thought this could be fun, and it was! Especially with songs that would make you laugh.
