Smoke
Smoke is an in-game indicator that a blockhead is getting extremely warm and may start taking damage.
Appearance
Smoke is grayish in color and square in shape. It drifts up and away from a blockhead, at first quickly growing and then shrinking. The rate of its generation is an indicator of how hot a blockhead is.
Where Found
Being too close to magma, found deep underground at the center of the world, usually within 25 blocks, can raise blockheads' temperatures to an extreme, causing smoke to appear. Other potential causes include dressing too warmly in a desert biome or even in a really warm climate, eating too many chilli peppers or coffee too quickly, and standing close to too many fueled crafting surfaces for too long.
Notes
The smoke given off by a too-hot blockhead is different from smoke emitted by campfires, fire, or torches. The latter is darker, and for uncontrolled fire is quite abundant.
A blockhead swimming submerged will not emit smoke no matter how hot they are.