People in the same family are related by blood and "blood is thicker than water" says that family ("blood") relations are more important than relations with friends. Example: "When my best friend and my brother got in a fight I had to help my brother; blood is thicker than water." "Blood is thicker than water" compares the thickness of blood (family relationship) to the thickness of water (friendships) and says that our family relations are more important ("thicker") than all others. Example: "Friends will come and friends will go but your family is always there for you; blood is thicker than water." Family relations (blood) are more important (thicker) than other relations (water) so "blood is thicker than water."