Horton says that there is indeed a person on his little clover, and that there are probably more of them, too. A family, even. And he doesn't want them to be disturbed.
The kangaroo is super mean, though, and just laughs at him. He calls him a fool, and Horton looks on concerned as they jump into the pool. Don't want those little tiny invisible people to drown, now.
Everyone's talking about Horton and how loony he is for talking to a little speck. Horton's solution? He walks around carrying the clover, afraid to put it down.