I've had several occasions to instruct students on machining techniques. One common demonstration is clamping. I can't determine why simple physics appear so foreign to them. It's quite easy to reinforce, and the point is not easily forgotten.
It goes like this.....
"Put your finger on the table" (Now, I use any convenient object but a pencil works best)
"Set one end on your finger, other on the table"
"What do you feel when I press here?" (the far end with firm pressure).
"How about here?" (nearer end, same pressure).
"Owww" (herein lesson could endeth)
"The length of these slots is a convenience, not tolerance". Put clamp studs close to the work as possible, OK"?
Between lathe and mill, mills are somewhat safer with parts of smaller mass, and injuries primarily waist-level and above. Lathes?, well, many of us recall shafts whipping, or castings in suborbital flight...