Use the up and down arrows to control the paddle on the left side of the screen. Notice the paddle on the right side of the screen moves in the opposite direction. Controlling one controls both. Move your mouse to the bottom of the stage to reveal a slider that can adjust the speed of the cat and change the level of difficulty.
Thank you to will_it_work, whose (currently unshared) project, using two paddles for pong that could be independently controlled, inspired this remix (where the actions of one paddle influence the other).