Click on Whismur for it to cry. Click the attacks to change the attacks. Click Feed to feed it. (It also gives you EXP to evolve, you need 500 to evolve, 50 to change its level. Each Pinap Berry gives 10 EXP to Whismur.) Click Uproar to, of course, make it cause an uproar, and scare the Mews.
The cry of a Whismur is over 100 decibels. If you’re close to a Whismur when it lets out a cry, you’ll be stuck with an all-day headache.