Not very technical, but try this one:
An airplane climbs because it has more power available than the amount of power it needs for straight flight at a particular airspeed. This simply means, for example, that if you are flying an airplane that has a normal cruise speed of 100 kts at full power and you keep it at 80 knots with full power, it must climb.
Grab you book and look for the power available vs. power required chart. Vy is the airspeed at which the difference between the two curves is the greatest.
With altitude, power available decreases. As you get higher, it decreases a lot. The point at which the greatest difference exists between the curves moves down the airspeed scale.
Vx is a little different (and takes some visualization). Once you have power available for a climb, it's how high can you bring the nose and convert that excess power into thrust pointing upwards, so that you climb steeper, not faster. An airplane with virtually unlimited thrust could fly straight up and the distance it can climb within a set horizontal distance be "infinite" (very loosely speaking). Think of our cartoon view of a rocket ship - straight up to outer space with no horizontal movement at all - forward airspeed is zero. (Of course, we don't fly airplanes that go straight up, but it helps visualize the concept.) Vx is the point at which you'll get the maximum excess thrust.
The same loss of power with altitude means that there isn't as much excess thrust available. So, you have the lower the nose. And that means airspeed has to increase.
As altitude increases and power and thrust both decrease, Vy will decrease because there's less power available. Vx will increase because there's less thrust available.
