Xerj,

I was trying to explain to a non-pilot why increased power is required with
altitude.

It is? I may have an idea what you mean, see point 2 below.

She said "isn't the air thinner up there so there isn't as much
resistance?"

Exactly right. Bright girl! That's why turbocharged piston airplanes like to
fly high.

I said "yes, but the plane needs to fly fast enough for the air
over the wings to feel like it does down low. So the speed required goes up
you get higher. More speed need more power."

Uhm, not really. For a normally aspirated engine, the power output will
decrease during the ascent because of thinner air, which means fewer air
molecules per volume to burn. Thus, you need to increase the power setting to
make up for that (and you need to put less fuel in the cylinder, too, which is
achieved by leaning). At some point you will run out of throttle doing that.
There is an optimum altitude where the balance between loss of air resistance
and loss of engine power amounts to the best speed for the maximum amount of
power available. It is usually between 6000 and 8000 feet for what we fly.

--
Thomas Borchert (EDDH)