Word of the day pyrophoric. Understanding this word means understanding using flint and steel can start a fire. I learned about it here, "Flint and Steel: What Causes the Sparks?"
Pyrophorics are substances that spontaneously ignite below room temperature, which is about 70°F (21°C). By spontaneous we mean these substances will simply catch on fire of their own accord without you having to do anything such as light it with a flame. This is also known as autoignition.