A person on the weather network just recited this piece of sailing lore.
Red sky at morning; sailor’s warning
The problem with this is that there are two variables, namely the time of day and the color of the sky. However because warning rhymes with morning, it focus on the time of day instead of the color. As a result, the color could end up getting substituted without changing anything, and thus ruining the mnemonic:
Blue sky at morning; sailor’s warning
Purple sky at morning; sailor’s warning
The mnemonic should rhyme the primary variable, for example:
Morning sky be red; sailors be dead.
(Of course in this specific case, the mnemonic actually is correct because the line she recited was part of a longer one in which the active variable is in fact the time of day:
Red sky at night; sailor’s delight.
Red sky at morning; sailor’s warning.
However if the quote was indeed just that one line, then it would need to be rewritten to focus on the active variable.)