Show Him/Her the "Hanky" test. : Take a hanky or a white cloth (even a sheet of toilet paper) then draw on a cigarette, place the cloth againt your mouth and blow out through the cloth. The stain will speak for itself.
Spot on @jprossouw . 1shot is the middle of the road, let's say round about 100-150ml a month even less.
All the "stick" vapers ( like me)who find com juice to expensive or sweet or for the heck of it.
Think of it in this way. You can go to the supermarket and buy the following; a baked cake, pre mixed cake flour or all the goodies to bake your own cake.
I see 1 shots as "pre mixed flour".