There are $n$ jellybeans in a bottle with unknown dimensions. What is the only surefire way of knowing how many jellybeans are in the bottle?
Perhaps by counting the number of jellybeans in the bottle.
You can empty the bottle and count the jellybeans on the table.
There is another way that is probably far less time-consuming, depending on $n$.
Weigh the bottle of jellybeans. Call this value $F$.
Pour out the bottle of jellybeans.
Weigh the empty bottle. Call this value $E$.
Weigh a single jellybean. Call this value $J$.
The number of jellybeans is
Eat all of the jellybeans because jellybeans are yummy. There are 0 jellybeans in the bottle.