A formal Experiment:

When I was twelve, my father took me to a science fair at a local college. One event was held in a large hall, about fifteen seats wide and maybe forty seats deep. The room’s floor angled up so that those in the back could see well. We sat in the front, where the ceiling was high above us.

The college students giving this experiment explained that they were going to show how you ignite iron filings from a relatively cool flame. They had a large flower pot with, they explained, iron filings and sulphur powder. They would use an ordinary match to light a magnesium fuse. The fuse would then light the iron and sulphur, and those would burn together.

They did this. An awesome, dense, yellow cloud ascended from the pot to the ceiling and drifted to the back of the room. As it approached the back, those in the back quickly exited the room. The cloud began to fill the whole room, and the awful rotten egg smell drove us all out; not, however before we looked around for the jokester experimenters. They were nowhere to be found.

That part of the science building was closed off for the rest of the evening.