Mercury, when in liquid form, is quite harmless. You could even drink it (when it's fairly cool) and it will pass through your system with little damage (NOTE DO NOT TRY THAT). The problem with mercury is when it starts to heat up. Most metals are solids at room temp, but mercury is not. When the temperature rises mercury vapors start to rise into the air. When inhaled, these vapors do terrible things to your body and brain.