Hemoglobin, a substance in human blood, transports oxygen from the lungs to the rest of the body  With each oxygen molecule it picks up, a hemoglobin molecule becomes more effective at picking up additional oxygen molecules until its maximum capacity of four oxygen molecules is reached
Grabbing an oxygen molecule changes the shape of the hemoglobin molecule, each time causing it literally to open itself to receive more oxygen
A hemoglobin molecule that has picked up three oxygen molecules will be more effective at picking up another oxygen molecule than will a hemoglobin molecule that has picked up only one oxygen molecule.