Thing One and Thing Two are two characters from The Cat in the Hat book. They were released from the box that the cat brought in to show to Conrad, Sally, and the fish. The things caused mischief by flying kites in the house which bumps the wall and also knocking down everything in their path. Conrad was able to stop the things with a net. They were also taken away by the Cat who put them back in the box. As of now they appear in the tv series The Cat in the Hat Knows Alot About That! voiced by Rob Tinkler.
At some point there happens to be a Thing Three and Thing Four.
In the film, Thing One is played Danielle Chuchran and Taylor Rice, and Thing Two was played by Brittany Oaks and Talia-Lynn Prairie but Dan Castellaneta provided the voices for both of the two Things.