Four rubber wheels, A, B, C  and D  are touching so that if one moves the others must also move. If wheel C turns in the direction shown, which way will wheel D  turn?

A square piece of glass is placed upon the floor and leant against a wall. Would it be most safely placed in position A, B or C?

A beam is supported at the points shown by the four arrows, A, B, C and D. Which two arrows could be removed so that the beam remains in the same position?