People make tea every single day without thinking at all about the scientific processes behind their favorite beverage, but if we closely examine what happens when we plunge a tea bag into a cup of boiling water, we can learn a lot about diffusion and how it works.
Diffusion is the process by which highly concentrated molecules spread out into areas where molecules have a lower concentration. In simple diffusion, this spreading happens naturally with no help from any other substance. The molecules that make up tea are highly concentrated in their tea bag. While the tea leaves themselves are held in place by the tea bag, the molecules of flavor and color can move through the bag, which serves as a membrane. These molecules, which are so highly concentrated in the bag, naturally seek to spread out, and they diffuse through the water, which has no molecules of tea in it at first, to balance out the substance.
The boiling water adds another element to this process of simple diffusion. When molecules heat up, they move faster, so the tea molecules become energized by the hot water and move much more quickly from the high concentration in their tea bag throughout the lower concentration of the water. Pretty soon, the tea has completely diffused into the water (which actually becomes tea!), and the process is complete. Notice, too, that some people prefer stronger tea and therefore let their tea bags steep longer so that the tea molecules will spread out more fully. Other people prefer weaker tea and remove the tea bag before the diffusion process is complete.