When Ash, Misty, and Brock run into the Pokemon Center because of the storm, they are standing in the doorway exclaiming about all the rain when Pikachu interrupts them by shaking his fur dry like a dog.
Everything Mew does, from messing with Meowth, to popping the bubble it saved Ash with.
While Mewtwo is gloating on how he is superior to Mew, Mew pays no attention and starts chasing its tail instead; at one point it even gives a look as if to say "Is this guy serious?"
At one point, Mew spots one of the big windmill-like turbines outside of Mewtwo's fortress. It flies up to one of the blades and sits on it. When the blade lifts, Mew falls onto the next one, then the next one and so on. Then there's an adorable close-up of it giggling in delight.
Mewtwo, the last character one would expect to have such a moment, has one in the original Japanese version; when attacked by Ash's Charizard, Mewtwo, not surprisingly, shrugs it off without a scratch and immediately quips "What a bad tempered Charizard". note Sadly, the English version goes with the much less effective "Your Charizard is poorly trained". A shame, especially since the English version could have easily delivered the original line and retained the humor.