Bob, Nora, and Clara are discussing a problem in which the pressure and temperature of the air in a bicycle tire were given. The question asked for the new pressure on a much warmer day when the volume of the tire was the same as before. In the book, the pressures were given in units of psi (pounds per square inch), and the solution used the equation P, =P and kept the pressure in units of psi. Part of the students dialogue is given below. Clara: This really confuses me. I thought that pressure and volume had to be in the right units. Nora: It s okay to measure pressure in units of psi because we were talking in terms of air in a tire, psi works better for the pressure unit. It s more convenient, Bob: I just assumed it was OK to use psi as units since pressure is on both sides of the constant volume equation, so the units cancel. Clara: Oh, yeah. That must be right. So the pressure could be in mm of Hg and it would still work, right? Bob: I think so. Nora: But then why did we have to convert the temperature from Celsius to Kelvin? Temperature son both sides of the equation, too. I still don t get it. In 2-3 thoughtful and complete sentences, help Nora out: Why is it OK to keep the pressure in units of psi (not m) but its not OK to keep the temperature in Celsius? If you are puzzled, please explain clearly what it is that puzzles you.