Spontaneous combustion can happen at any time, anywhere.
Firefighters determined it caused Monday's fire at Pop's Pizza in downtown Quincy.
"Spontaneous combustion is a fire that started from a product that self heats and that can happen through any kind of chemical process where some kind of natural material is being broken down and decomposing," Michael Dade with the Quincy Fire Department said.
Because it's spontaneous, you can't predict when this type of fire will start, but you can be aware of items in and around your home that could self ignite.
"You definitely have to be mindful of how you store your cleaning supplies and cleaning rags. A lot of times, we've seen where people have oily rags after treating wood, or something like that, and they've washed them and think, hey, these rags are clean, but there's still a lot of oil left and it's not totally clean," Dade said.
Dade says spontaneous combustion can occur with items that have a low ignition temperature, like grass, hay and cloth such as rags.
Click here to find out more about spontaneous combustion.
"When they have a low ignition temperature, that chemical process that is breaking down those natural materials gives off energy. And that energy is heat and when those items are piled on top of each other, and that heat can't release, you have your ignition source. It could start a fire," Dade said.