I like to get a small water ballon, fill it ALMOST full, put about 2 or 3 drops of RED food coloring (keep adding more until it's blood red), get a pin/small tack and poke a whole in the side of the water baloon, and while whoever you want to prank is sleeping trickle/sqirt the water from the ballon anywhere where it will look like the person is bleeding (I personlly put the "blood" around the nose area it looks REALLY realistic)!!
It works ALL the time!! I have even fooled MYSELF when trying to get some ready I spilled somw on my hand! XD