Beginner puzzle
This puzzle is intended to be suitable for people who are new to puzzle solving.
Clarification: Both experienced solvers and new solvers are welcome to post solutions to this puzzle.
Sixteen counters, which are black on one side and white on the other, are arranged in a 4 by 4 grid. Initially all the counters are facing black side up. A single move consists of choosing a 2 by 2 square within the grid and flipping all four of its counters over. Describe a sequence of moves of minimum length that finishes with the colors of the counters of the 4 by 4 grid alternating (as shown in the diagram).
This puzzle is from a UK Junior Mathematical Olympiad.