I like to explain it by the "electric fence". the ball has to touch the line. If it gets 'shocked', it is in.
There is a classic argument about the ball that lands out but compresses and touches the line. some want this to be 'out' because the ball is a sphere and was 'out' before it touched the line. My response to this argument is: You are absolutely right...in physics class, but this a a sport and the 'rule' makes this ball in.
