Who Chocked Up the Grizzly with Laughter?

The Grizzly is a moody animal who doesn't know how to laugh. That's why he's won the no-laughter cup in the zoo which is given to the animal, who resists tickles the best. But that morning, the Grizzly gets a letter that makes him laugh uproariously. Who did it? At the same time, Hermione has taken a day off and Sherlock will have to investigate with a totally idiotic partner: the Kangaroo!