Did you know the classic puzzle of making 4L with just a 3L and 5L jug? Using only a couple of fixed-size jugs, repeat filling, emptying, and pouring to make an exact target amount.
This puzzle is famous enough to have shown up as a bomb-defusing problem in Die Hard 3. It turns out every amount you can make with two jugs is exactly a multiple of the two capacities' GCD — if the GCD is 1, you can make any whole-number amount, but if the GCD is 2 or more, you can only make its multiples.
There are only three moves: fill (fill a jug all the way), empty (empty a jug completely), and pour (pour from one jug into another until the receiving jug is full or the pouring jug is empty). Using just these three, make the target amount exactly in any jug to win.
Drag a jug onto another jug to pour directly. The pour buttons do the same thing.
Stuck? Click the hint button. It tells you the single fastest next move toward the target from where you are now.