Early one Halloween evening, Frank's friends asked him to join them in smashing their neighbors' decorative pumpkins. He complied. Later that night, he was surprised by his own failure to resist them when they encouraged him to throw eggs at passing police cars, for which he was caught and punished. Frank's experience best illustrates the:

In psychology, foot in the door phenomenon refers to a situation that make people becomes more likely to agree to fulfill bigger request after previously fulfilling smaller request.

This happen because after fulfilling a small request, most people developed a positive feeling from the experience of helping others. It make us feel good and somehow valuable. When we're asked to fulfill bigger request under this emotional condition, we become more likely to agree with it.

This is what happened in the example above. First, the friends ask to smash neighbor's decoration (smaller request). After that, he became more likely to agree to throw eggs at police cars (bigger request)