A staircase.
A bar of soap.
Mount Everest.
A pit.
Only three times. First, you put seven marbles on each side of the scale. If they are equal then the marble left out is the lighter one. If not, then take the lighter side and put three of those marbles on each side of the scale. If the scale is level that means the marble left out is the lighter one. If not take the lighter side and put one marble on each side, if it is equal then the marble left out is the lighter one. If not then the side that is lighter is the marble you're looking for.
A son, his father and the grandfather went fishing. The father and the grandfather are both dads. The son and the father are both sons.
The catcher and umpire (Jack is playing baseball)
A barber.
A key.
Nobody the three doctors are girls.