Demonstrate shaping techniques used to reduce fin drag. Install the engine mount for their rocket. Even a young child knows that it's the fire squirting out of the bottom of the rocket. That's not really rocket science. And it doesn't take a rocket scientist to realize that in order for the rocket to get off the ground, the thrust of the engine must be great enough to first overcome all the forces tending to hold the rocket down, then accelerate it from rest to a very high velocity.
Lift is generated by the flow of air over a rocket's fins.
The angle at which the fins meet the air stream determines the amount of lift created. The greater the angle, the greater the lift. As the amount of lift increases, so does the amount of drag. With a little thought, you can probably see that these forces are not going to remain constant during the flight of the rocket. The fuel on board the rocket is going to burn, causing the total weight to grow ever smaller until all the fuel is used up.