A. Because the air is filled with vapour, and the humidity penetrates the wick; where (being formed into steam) it expands suddenly, and produces a little explosion.
Q. Why does a drop of water sometimes roll along a piece of hot iron without leaving the least trace?
A. If the iron be very hot indeed, the bottom of the drop is turned into vapour, before the drop can evaporate; and the vapour thus formed buoys the drop up, without allowing it to touch the iron at all.
Q. Why does it roll?
A. The current of air (which is always passing over the heated surface) drives it along.
Q. Why does a laundress put a little saliva on an ironing-box to know if it be hot enough?
A. If the saliva sticks to the box and is evaporated, the box is not hot enough; but if the saliva runs along the box, it is.
Q. Why is the box hotter if the saliva runs along the box, than if it adheres to it till it is evaporated?
A. If the saliva runs along the box, the iron is hot enough to convert the bottom of the drop of spittle into vapour; but if the saliva will not roll, the box is not hot enough to convert the bottom of the drop of spittle into vapour.