Goat, Cabbage and Wolf
A farmer is to ferry across a river a goat, a cabbage, and a wolf. Besides the farmer himself, the boat allows him to carry only one of them at a time. Without supervision, the goat will gobble the cabbage whereas the wolf will not hesitate to feast on the goat. Clickable are the three shapes by the river and the arrow that indicates the position of the boat (and, of course, the farmer.)
As of 2018, Java plugins are not supported by any browsers (find out more). This Wolfram Demonstration, River Crossing Puzzle, shows an item of the same or similar topic, but is different from the original Java applet, named 'OldPuzzle'. The originally given instructions may no longer correspond precisely.