Transcranial direct current stimulation (tDCS) is a safe and non-invasive method of temporarily altering the activity of neurons by passing weak currents through electrodes on the scalp. It can enhance mathematical skills, memory, attention and language learning. Richard Chi and Allan Snyder of the University of Sydney in Australia wondered whether it would help people solve brain-teasers. They began by training 60 volunteers to solve arithmetic problems expressed in Roman numerals constructed from matchsticks.