PopCorn is played with the keyboard or mouse, the latter being the preferred
method. Keyboard is however the default setting. The default keys are:

Paddle left             : J
Paddle right            : K
Acion                   : Space bar

You can assign different keys by pressing F5 in the main menu, but the game
will not store your selections -- if you exit the game and start again, the
keys will revert to the default settings.

To switch to mouse control, press F3 in the main menu. Press F4 to switch back
to the keyboard.