Distinguish between goods (things you can hold) and services (actions people do for you).
People exchange two main kinds of things in an economy: goods and services. A good is a real object you can hold, like an apple, a book, or a bicycle. A service is an action someone does for you, like cutting your hair or teaching a class.
A helpful test: can you hold it in your hands? If yes, it is probably a good. If it is something a person DOES for you, it is a service.
Mastery looks like: They can correctly sort real examples into goods and services and explain the difference using the "can you hold it" test.
Common stumbles: Confusing a service with the object involved in it (for example, calling a haircut "a good" because scissors are used).