It was a cacophony of ringtones and languages. He heard Indian, Farsi and passionate Spanish. A traditional nokia rang every few minutes somewhere behind him, neglected by its owner. In that same area someone was now yapping at her phone which she had fished all the way from the depths of her purse just moments ago, when it started asking for attention with its tinny ringtone. And all this on the day he forgot his headphones. In theory this hour of commuting back and forth was useful for finishing his homework, but in practice this train was always full, usually with even more noise than today. Next to him a student was texting busily over his opened book. And he even had headphones. He squeezed his eyes together and bend down on his notebook. Come on, push through. Mass divided by density is..