Suppose a student gave two mice water alone and incubated one for one day and the second for six days. the first mouse maintained its mass, and the second mouse died on the sixth day. what is the most likely explanation?

The reason why the second mouse died is because, the water that the mouse was given was not enough to sustain it for six days.
Living things need energy to carry out and maintain the biochemical activities that occur continuously in their cells.  This energy is derived from the food and the water that are consumed. For a living thing to remain alive and maintain good health, it must have a constant supply of water and food on a daily basis.