Most people who need a resistor don't need to know how they work, just why they need it.
There is a difference between an electrical engineer and a physicist.
Most people who need a resistor need to know which resistor will work for the circuit intended, and what grade of resistor will not burn up in the circuit. Most people doing so, will already know coming into the store what they need. In order to be able to pick the right one, you need to know a bit about how they work and how they are specified. They have to have the right resistance and be for the right amount of current/voltage in the particular circuit. You wouldn't put the cheap 5 cent plastic ones in a very high voltage or amperage circuit where a ceramic coated one is needed, depending on what you are doing, unless you specifically want it to fail catastrophically, even if they have the same resistance. Not knowing how it works and the right application for the right resistor could cause a fire. The customer should already know some of this, but if the fool behind the counter is going to try to help, they should know the difference as well, so they don't get sued for pushing the wrong part on a customer without checking first. I wouldn't leave the store with the wrong part, but if they won't let me look for the right part in their bins, I will leave with no parts and no sales to them.
No, that's why he needs someone in Radio Shack to help him.
Actually, I do know. It would also be nice if the person at the store knew what they actually are and where to find them, and how to get the right one, if they are going to be so damned helpful. Knowing a little of how they work, and why you would want one (or more) for a project you are working on would allow them to help spec the right one depending on the grade of work being done by it in the circuit that it will potentially (pun intended) be put into. If they can't take the time to learn about their products, or if the manager's can't hire people who already know, then they should just leave the people rushing to the back of the store to the parts section alone, and just go over to talk to grandma and grandpa looking at the Trac phones.
Do you know how a resistor works?
Do you suffer painful illumination? -- Isaac Newton, "Optics"