But how often does the lamb shake its tail?!

What an anxious/impatient youngster would logically want to know in instances when his parent promises a certain action/food/relief/entertainment "in two shakes of a lamb's tail".

Cranky child: "Are we there yet?!"
Parent driving (cheerfully): "Oh, no worries --- we'll be there in two shakes of a lamb's tail!"
Cranky child: "But how often does the lamb shake its tail?! Oh, sure --- a NORMAL AND ALERT lamb probably "flutters its little stumpy thing" quite regularly, but if the lamb is asleep or not feeling chipper, its behind-flipper might not move much at all, and so it might take HOURS for it to jiggle even ONCE, let alone TWICE!"