it isn't spread, it develops from other causes
A concave mirror causes light to converge to a focal point, while a convex mirror causes light to diverge and spread out.
When one part of a vibration causes air particles to spread out, they form ararefaction
That's how they spread their seed.
rising molten magma
When one part of a vibration causes air particles to spread out, they form ararefaction
The shortening in them melts and causes them to spread.
the spred of bactriea is caused by sickness
foam
Malaria
Convection currents
The motion of the water molecules eventually causes the flavor to spread.