They are more apt to look at it carefully instead of feeling they can draw it from memory. Bent cans are good. Toys are good. Food is good. Trees are good.
Mirrors are good.
Be sure they are slow, careful, and deliberate using continuous line, not sketchy lines. Do NOT show them your work. Do NOT demonstrate to them how you do it. Do run your finger very slowly along the edge of the thing the student is drawing. Do encourage slow, concentrated looking.