Have you ever been taking a shower when the smoke alarm goes off? Well, sometimes the steam from a shower can actually activate a smoke alarm. You see, steam is airborne particulates. In other words, steam looks like a cloud to us, but it's actually millions of tiny water particles floating in the air.
Smoke is very similar. What appears to be a dark cloud to our eyes is actually made of millions of tiny particles--in this case, not of water, but of burnt matter. There are two kinds of smoke detectors: ionization alarms and photoelectric alarms.
They work in different ways, but both are designed to flip a switch when they come into contact with airborne particles. A sudden cloud of steam may trigger a photoelectric cell looking for particles as quickly as a cloud of smoke.