You Can't Lick Your Elbow
It's physically impossible to lick your elbow. The reason being that from any angle that you take your tongue is instantly too short to reach the tip of your elbow. There has been known claims of people possessing the talents to do this task.
Planck Length Is The Smallest Unit of Measurement
If you want to find the very smallest things around, you'd need to track down something that's the size of the Planck length. It's the smallest possible size of anything in the known universe at a mere 1.6 x10-35 meter across. That is the equivalent of around a millionth of a billionth of a billionth of a billionth of a centimeter.