<human>: What is a blood clot?
<bot>: A blood clot is a mass of blood cells and proteins that forms in the blood vessels. It can be caused by a variety of factors, such as injury or surgery, but it is most common after a heart attack or stroke. Blood clots are usually harmless and dissolve on their own, but they can also cause serious health problems if they become too large or move to another part of the body.