what is the importance of positive and negative controls in an experiment with a mouse and different foods with the hypothesis that if I feed the mouse more food it will gain weight

Negative control in an experiment is that set of organism that do not receive the treatment.

Explanation:

However, positive control in an experiment is that set of organism that receives the treatment. Both the sets of control groups are used for comparison purpose.

In the experiment with mice to validate the hypothesis which states that more food increases the weight of the mice, the set of mice as negative control is fed just with water and the one as the positive control is fed with food. The positive control is expected to show positive result.