Unlike prayers, charity, and pilgrimage, fasting is an invisible act. Only Allah and the person who is fasting know whether he or she is fasting or not. One may quietly eat or drink something and no one will notice and no one can find out. However, the fasting person has made this commitment for the sake of Allah and he or she wants to guard the purity of his or her fast for the sake of Allah. Fasting thus teaches sincerity, and it helps a person learn to live by the principles of his or her faith regardless whether others know or do not know. This is the very purpose and essence of taqwa. - Islamic Thinking :)