Metaphors can easily pull you off the straight and narrow.
Sexism: on why promiscuity amongst men is acceptable but not amongst women. A key that can open any door is a good key but a lock that allows any key to open it is a bad lock.
Racism: on national identity. Just because a cow is born in a stable, it doesn’t mean it’s a horse.
Ageism: When my screwdriver is worn out I throw it away.
Though metaphor isn’t all bad. Consider a favourite from John Donne.
Our two souls therefore which are one…
If they be two, they are two so
As stiff twin compasses are two.
Thy soul, the fixed foot, makes no show
To move, but doth, if th’other do.
And though it in the centre sit.
Yet when the other far doth roam,
It leans and hearkens after it,
And grows erect, as that comes home.
Such wilt though be to me, who must
Like th’other foo obliquely run,
Thy firmness draws my circle just,
And makes me end where I begun.
Which has all the controlled serenity of a Bach fugue.
Not always rubbish.