The oldest known jigsaw puzzle is a map of the world created by John Spilsbury in 1767. He mounted a map on a sheet of wood and used a jigsaw to cut it into small pieces. This was a tool used to teach geography to children, but it also became a popular pastime.
The hardest jigsaw puzzle to solve can vary depending on the person's skill level and the difficulty of the puzzle. Some of the most challenging puzzles include those with highly detailed and complex images, such as puzzles that feature a large number of small pieces or irregular-shaped pieces. Some puzzles with thousands of pieces or ones with repetitive or similar pieces can be considered hard to solve.
The 15 puzzle is considered unsolvable because it's impossible to put it in its solved position, starting from a random configuration. The puzzle consists of 15 sliding tiles, each with a number on it, that are placed on a 4x4 grid. The puzzle is solved when the tiles are arranged in numerical order. However, due to the specific configuration of the puzzle, not all random configurations can be solved.
The hardest puzzle on earth can be considered subjective as it can vary depending on the person's skill level and interests. Some people might find a Rubik's cube to be the hardest, while others might find solving a mathematical equation or a crossword to be the most challenging. The most difficult puzzle can also depend on the level of complexity, a combination of different types of puzzles or a time limit to solve it.