Power.

This evening as part of the change I got for spending a twenty I received a ten dollar bill with the serial number pretext BTU. This struck me as very appropriate. BTU is short for British Thermal Unit, which is a measurement of energy. A bill is the monetary equivalent of a unit of energy. We use dollars to indicate the strength of the economy, the value of transactions, and so on, just as a BTU is used to measure the heating capacity of a heater, the cooling ability of an air conditioner and so on .