Liars often behave in subtly abnormal ways because they are under unusual emotional strain and mental stress to make the lie sound believable, and it's these abnormalities that can give them away.
For example, a liar might be nervous about lying, or angry about the truth. These emotions are difficult to hide, and might reveal themselves in nervous tapping of the feet, or clenched fists.
A liar often tries to suppress behaviour that they think might give away their deception, which is why, liars will probably look you in the eye rather than looking at the floor, and try to avoid making what they think are irrelevant movements with their hands and fingers.