IIT-Madras' Prof Mahesh Panchagnula explained that the practice of sprinkling water on hot tawa before pouring dosa batter is linked to Leidenfrost Effect, identified in Germany in 18th century. When tawa isn't hot enough, the water evaporates slowly, but at the right temperature, water glides and floats on thin vapour layer. This prevents dosa from sticking and makes it crisp.