The cost of something doesn’t pertain only to its immediate monetary value. The cost of going to Karate lessons is not just £15 an hour, it is also the 20 minutes spent getting there, the hour in the class, and the 20 minutes getting home. You could spend that 100 minutes watching TV, cleaning the house, or learning a language. Knowing the opportunity cost of any decision helps us better evaluate it. A free ticket and paid travel to a concert 2 hours away aren’t cost-free. Intuitively, we know this. Knowing why it isn’t entirely free helps us recognise the alternatives and make an informed choice.