so, this is a tutorial on the /execute command released in minecraft vanilla 1.8
you can do lots of things with this command,
/execute @e[type=Arrow] /summon Cow
and /execute @e[type=Cow] /summon PrimedTnt
if you copy/paste BOTH of these commands into two different command blocks on a redstone clock, then it will create an "Exploding Cow Bow" what this does is, it waits until you shoot an arrow from a bow and when a cow is spawned. The Arrow Will Summon a Cow, and the Cow, will summon Tnt! Pls do try this, it works in version 1.8 and over.