“Okay, which one of us do you think it getting the placebos?”
Placebo (n) /pləˈsiː.bəʊ/ a substance given to someone who is told that it is a particular medicine, either to make them feel as if they are getting better or to compare the effect of the particular medicine when given to others.
Given the question, we can guess that the two mice are in a laboratory. Here, the mouse that asks the question seems quite happy to realize that, compared to the other mouse, its body has not been influenced in a strange way.
Read this post about control groups to understand how scientists carry out many experiments with different groups.