A fire alarm usually detects smoke in case of fire. Where should such an alarm be placed in a room?

As we know that the smoke created by the fire is hot air and we know that hot air expands (volume increases) but have same mass as before and it becomes lighter than the ordinary air, so the light air floats up in the sky and rises up, so due to this reason the suitable place for the sensor would be the ceiling.

Warm air rises up cool air sinks down. ALWAYS REMEMBER