If Mush-Mush had Bolly's speed, he'd want everyone to see just how awesomely fast he was. But Bolly is different - he just likes to use his speed helping Mushables. So when Mush-Mush organizes a big stunt for Bolly, he refuses. He just doesn't like to show off. But Mush-Mush promised everyone a show so a show he will give them with Mush-Mush performing instead of Bolly! Gulp!
