I think I was in second grade when I learned this. I was definitely in elementary school. This is not even eighth-grade grammar.

A period ends a sentence that is a statement (a telling sentence).
Examples:

• I am going shopping.
• Sally likes Italian food.
• The sky is blue.

A question mark ends a sentence that is a question (an asking sentence).
Examples:

• Are you going shopping?
• What kind of food does Sally like?
• What color is the sky?

This week I witnessed an executive and a teacher confuse the two. I wept.