It seems that understanding numbers in physics is often almost as important as understanding concepts, specifically when approximations to equations are made. But I found myself lacking in intuitions in these numbers.... When my professor is lecturing, he always gives out numbers. for instance, kT for room temperature is about .026 eV (yes, he just remembers that) and rest mass of electron is about .51 MeV. From these numbers, my professor deduces how some approximations are valid. (When quantum mechanics can be applied, and/or a certain equation is valid) It seems that I have a hard time grasping what those numbers really mean. I know it is a vague question but is there any way to get a better intuition out of numbers and/or numerical constants? Like when we say the mass of a proton is around 938 MeV, what does that tell us? In addition, do most of you guys memorize important physics constants/numbers? like the constant G, h, k.. etc? in different units?