Question

what is the outcome variable (dependent variable) in this experiment?
amount of water in test tube

Explanation:

In an experiment, the dependent variable is the one that is measured or observed and changes in response to the independent variable. Here, the amount of water in the test tube is being measured over time (as different days pass) and likely changes based on the type of nail (the independent variable).

Answer:

The amount of water in the test tube.