Adaptation is the adjustment of the sensitivity of a sensory system to the level of stimulation it gets. You experience it frequently. You enter a darkened movie on a bright afternoon. At first, you can't see anything except the lit screen; in 10-15 minutes you can see the people in the seats. The eyes have increased their sensitivity to fit the low light level.

All senses adapt. The nose adapts a lot. When you walk into a house with a musty smell, it is unpleasant at first, but soon you no longer smell it. Temperature adaptation can give the odd effect that the same water can feel warm and cool at the same time. Put lukewarm water in one pan, cold water in another, and hot in a third. Put one hand in the hot water and the other in the cold for a few minutes. Then put both hands into the lukewarm water. It feels warm to the cold hand and cool to the warn hand. The diagram below illustrates this illusion.

Taste adaptation can even change the taste of another stimulus. Putting strong salt solution of the tongue for a while makes pure water taste sweet