Romans 12:20 

"Therefore if your enemy is hungry feed him; if he is thirsty give him a drink; for in so doing you will heap coals of fire on his head"
In Bible times people carried objects on their heads eg jars of water, food etc. People also had for warmth and light cooking, braziers. If fire ran out, they would simply go to their neighbors to get fire. Then place the brazier back on their heads. If the neighbor was generous he would also add coals. Therefore feeding and giving drink to our enemy are acts of generosity. Source: Strange scripture that perplex the Western mind by Barbara M Bowen