The study looked at how the amount of moisture in coal affects its tendency to catch fire on its own. By testing 55 coal samples with different moisture levels, researchers found that coal is most likely to burn quickly when moisture content is below 20%. However, having more than 20% moisture can actually prevent spontaneous combustion because the heat produced by burning is used to evaporate the excess moisture. This was confirmed by testing 15 more coal samples from a different area. The results show that the amount of moisture in coal plays a crucial role in determining its risk of catching fire spontaneously.