Cartilage is extremely important for the healthy functioning of a joint, especially if that joint bears weight, like your knee. Imagine for a moment that you're looking into the inner workings of your left knee as you walk down the street. When you shift your weight from your left leg to your right, the pressure on your left knee is released. The cartilage in your left knee then "drinks in" synovial fluid, in much the same way that a sponge soaks up liquid when put in water. When you take another step and transfer the weight back onto your left leg, much of the fluid squeezes out of the cartilage. This squeezing of joint fluid into and out of the cartilage helps it respond to the off-and-on pressure of walking without breaking under the pressure.