Calculating a stone's average speed during its fall is possible because average speed is defined as the total distance traveled divided by the total time taken. During the fall, the stone accelerates due to gravity, which means its speed changes continuously. Therefore, while average speed can be determined over the entire distance of the fall, instantaneous speed at any given moment would require more complex calculations. This distinction is crucial for understanding motion under gravity. Accurate explanations can help in teaching concepts of physics effectively.