Blood clots are clumps of blood. If someone has a cut on their skin, the blood will form a clot over the cut so it stops flowing out of their body. Blood behaves this way inside the body, too. If blood stays still for too long, it may form a clot. The heart pumps blood throughout the body and keeps it moving. Certain health problems may cause someone’s blood to pool and form a clot. When this happens, it can form a clot. A blood clot is dangerous because it may stop the blood flow to parts of the body or brain.