One of the annoying side effects of being absorbed in a gripping novel is that the cup of tea on the table becomes cold! Unfortunately, the tea would not heat itself by absorbing the heat around it, just as pieces of a broken egg would not put themselves together or milk mixed in coffee would not separate by itself. Such things are irreversible, and define a fixed direction of time—that from the past towards the future. This apparent progression of time is called the ‘arrow of time’. In a recent study, Prof Mahendra Verma of the Indian Institute of Technology Kanpur, suggests a way, different from any previous ones,  to determine the direction in which time is progressing. He uses the concept of energy cascade to define the arrow of time.