Jack tries to place magnets on the door of his refrigerator. He observes that the magnets don't stick. He guesses that the door of the refrigerator is made of a nonmagnetic substance. What stage of scientific investigation does his guess represent?

Observation stage and Hypothesis stage.

Before investigating, of course, you need to first make an observation. It can stem from any event. It is the part of the investigative process where you ask your question. You cannot start one without identifying the problem or the question that needs to be asked and you can do this through observation.

Hypothesis comes next after observing. You will make a hypothesis or an educated guess based on the observation that you just made.