This happened in the eighth or seventh standard; I don't remember exactly. The English teacher asked a student to wipe the board for her.

The student began wiping the board from left to right, as most of us would do it; something like this:

This resulted in the dust spreading in the rightward direction and a student who was standing just next to the board had to face a cloud of dust.
Noticing this, the teacher immediately asked the student to stop wiping the board and showed him the correct method to wipe the board: move the duster downwards, something like this:

After the incident, the teacher told the class:

"In life, always strive to do ordinary things, like cleaning the blackboard, in an extra-ordinary way. This will make your life extra-ordinary."

This has been one of the best pieces of advice I got. Following this has never let me down.