...Vengeance is mine; I will repay, says the Lord. but if they enemy
is hungry, give him food; if he is thirsty give him drink; for by so
doing thou wilt heap coals of fire upon his head....
Back in the day, way back in the day, even before Jesus' time, the nomads carried their fire with them. Fire was that important. Fire was too hard to start over, every time one wanted to leave or have light at night. Hence when it was time to move on, the coals from the fire would be put in a heatproof container to carry. Where was the container itself, carried?
On the top of their turban (or whatever their head covering was called).
Actually, the verse is telling us to be kind to our enemy. Kill your enemy with kindness. Think of the coals of fire as giving shame to your enemy. The purpose of the Christian is to bring his enemy to repentance and better conduct.
Besides, the verse tells us that "vengeance" is God's. Just be kind to your enemy and don't worry about how to extract justice from him. That could be dangerous.
Don't play with fire.