The price of US crude oil just went negative for the first time ever. Here's what that really means, and why you can't fill up your car for free.

· For the first time in history, near-term contracts for US crude oil traded negative on Monday, falling to about -$38 a barrel. 
· The coronavirus pandemic has cratered demand for fuels like gasoline, dragging demand for oil down with it. Storage is filling up fast as a result. 
· With nowhere for oil to go, the price is...