when they are hard they are soft

Mum walks into the bedroom to find her daughter in fits of
tears.
“Oh mum, it’s so unfair. Yesterday, Derek said he’d buy me a
diamond ring if I stayed the night with him. So I did, but all
he bought was a cheap trinket.”
“My darling,” says Mum. “Always remember this and you
won’t fall into the trap again. When they’re hard they’re soft
and when they’re soft, they’re hard.”