Blood is a red liquid that your heart pumps through your body. Blood carries all the nutrients that your body needs to live and grow. Normally you can never see your blood as it is inside you, but when you have an accident, cut yourself and bleed you can see your blood.
However, not all creatures have red blood. Some marine creatures such as horseshoe crabs and many insects have green blood, which contains the copper-based cyanoglobin rather than the red iron-based haemoglobin.