Try standing up to stretch your legs and a crack races right between your legs???
And the "CRACK" that sounds like a rifle shot seconds before it appears....
OR.....sitting in a folding camp chair, you hear a pop (you learn the sound differences over time) that sounds like someone slammed a sledge hammer down, and you feel yourself drop 1-2 inches, depending on how farup the ice was pushed before it cracked. That one NEVER fails to tighten up the sphincter!