6 glasses, 3 full and 3 empty ones are lined up as is shown.
Make the glasses alternate full-empty-full-empty-full-empty by moving only ONE of them.
Which one it should be?
Explanations
Take the 2nd glass in the row, pour its content into the 5th glass and then out the emptied 2nd glass back to its spot. Now all glasses alternate in the “full-empty“ order.