Bumpsandbruises Bruises occur when small blood vessels under the skin crack or break (by a fall, shock..). The blood passes into the tissue, and colors black and blue. It heals itself. It takes 2 to 4 weeks. The spot will change color, the degraded hemoglobin is converted into yellow pigments.
Cold relieves pain and reduces swelling and inflammation.
No ice or cold water at hand? Hold the hammer with which you've knocked on your finger on the sore spot. Metal conducts heat well. A large hammer takes more heat than a small one.