Time.
We only have so much of it available to us, and that amount is (regardless of what we may think) is finite.
There is a great movie called ‘In Time’, staring Justin Timberlake, where the premise is time is a commodity that can be bought / sold / traded / shared / stolen.
How ironic that I should think of that movie while stuck in an airport when my flight is one of several that have been delayed here today.