If you were walking along the street and found a bird lying on the ground, how would you react? You would probably poke the bird to see if it is alive. We have a peculiar habit since we are children of poking living things that we see for the first time. Through poking, we discover whether it is alive or dead, soft or hard, slimy or furry, docile or aggressive.
Prompting a reaction and observing the reaction is a surprisingly useful way of learning. In chemistry, we react an unknown substance with other chemicals to discover its identity. In medicine, we stimulate parts of the brain with electricity to discover what each part does. In physics, we build giant accelerator to crash particles together to find out their constituents and properties. If you fell into a cave so dark you cannot see even one foot ahead of you, the best way to find out if there is a wall or a hole or water ahead of you is throwing a rock in that direction.
This principle can be applied to psychology. To learn how people around you behave, provoke them. Human beings are extremely sensitive to stimuli and even when they consciously try to hide it, they will subconsciously react. If you keep (subtly) poking the person, you will soon be able to predict how they will react to something, what actions they will take, and you may even discover what is on their mind.
We cannot see the wind, but we can infer that it exists because the leaves blow. The best way to prove something that you cannot see inducing and looking for reactions.
