In 2014, Action Comics #1, a comic book with the first appearance of Superman, was sold for about $3,200,000. This comic book was originally sold in 1938 for $0.10. How can we measure how the value of Action Comics #1 changed since 1938? Many items increase or decrease in value over time. One way to express this is by using a percent. The new price is expressed as a percentage of the old price. For example, if the new price is 50% of the old price, this means that the new price is half the old price. If the new price is 200% of the old price, this means that the new price is double the old price. One example of an object with a value that could either increase or decrease over time is a house. Many factors determine the value of a house: market, location, nearby schools, appearance, condition, etc. If any of those variables change, the value of the house could go up or down. Use the interactive below to explore this idea.