• Smoke

An evening with a candle.

Smoke indicates the flow of air which can be described and modeled using the Navier-Stoke's equations.

They can be used to calculate the position changing with time for a set of particles or elemants that follow Newton's laws of motion where energy and momentum are conserved.

(i.e. from the 1^st law 'A body will continue in a state of rest or uniform motion unless external forces act upon it')