Another fun one to really mess with people:
#define true false
In my example code I create the defines:
#define STATE_LED_OFF 0
#define STATE_LED_SWITCH_ON 1
#define STATE_LED_ON 2
#define STATE_LED_SWITCH_OFF 3
So whenever you use STATE_LED_OFF, the preprocessor replaces it with 0 as that is what is specified in the #define. For STATE_LED_SWITCH_ON it replaces it with 1 etc.
The syntax is:
#define A B
and it equates to:
When you see A, replace it with B