flair / flare
A knack versus a burst of flame or a widening.
A flair is a knack: a flair for timing. A flare is a burst of flame, a distress signal, or a widening: the flare of a match, flare the trousers. Do not write flare for the knack.
Examples
Usea flair for numbers
Usea warning flare
Avoida flare for numbers