My students seem to understand it best when I put it like this:

Let's say an airplane stalls at 25 mph.
That means it "needs" more than 25 miles per hour worth of air molecules hitting it to be able to fly.
There are less air molecules in less dense air, so the airplane has to fly faster to "feel" 25 miles per hour worth of air molecules at high density altitudes.
So the air speed indicator indicates 25 miles per hour, but the airplane is traveling across the ground, (ground speed) faster.

Oversimplified, but it seems to help them grasp the concept.

Note: At "ISA" or sea level, 29.92 barometric pressure, and 59 degrees, with NO wind, 25 mph on the airspeed indicator would actually be 25 mph ground speed, (if the air speed indicator was accurate).

AV8TOR