1. Rubik's Slider
   The 15 Puzzle
2. Tic Tac Toe

It took its name from its 4-by-4 grid with 15 tiles and one empty space.

1. square
   triangular
2. circular

The pieces of the 15 Puzzle were all squares. Can you imagine trying to slide interlocking triangles?

1. all of them
   half of them
2. all except one

Exactly half the possible arrangements of the numbers can be returned to numerical order with the empty square at the end.

1. 1880
   1980
2. 1890

The 15 Puzzle was a huge sensation in the early months of 1880. Though the craze died down by the summer, the 15 Puzzle is still popular today.

1. Sam Loyd
   Noyes Chapman
2. P.T. Barnum

Though Sam Loyd claimed be its inventor, the 15 Puzzle was actually the creation of postmaster Noyes Chapman.

1. by jumping over another piece
   by flipping them
2. by moving them into an empty space

Sliding puzzle pieces can only be rearranged by moving them into an empty space.

1. True
2. False

Some very simple sliding puzzles are extremely difficult to solve.

1. It had no frame.
   It used rectangular pieces.
2. The pieces were all the same color.

Rectangular pieces complicated the solution of the Pennant Puzzle, also known as Dad's Puzzle.

1. True
2. False

The myriad of movement options and variables of these puzzles are so complex that they present a challenge to computers.

1. T-shaped pieces
   L-shaped pieces
2. Z-shaped pieces

The L-shaped pieces of Ma's Puzzle added a new degree of complexity to its solution.

1. moving two pieces at once
   converting the puzzle to a mathematical formula
2. There is no key.

Sliding puzzles are difficult because there's no key or formula that tells you how to solve them. As with most things, patience is your best asset.

1. taxi dispatcher
   warehouse manager
2. airline pilot

Sokoban is the Japanese word for "warehouse manager." In this puzzle, the manager (player) has to move boxes into their proper position.

1. Pieces can be pushed, but not pulled.
   The pieces are different colors.
2. The game has a warp factor that randomizes the tiles.

Because the boxes can only be pushed and not pulled, it's easy to reach a dead end.

1. mathematicians
   philosophers
2. psychologists

While philosophers and psychologists may find these diversions fascinating, the principles behind solving sliding puzzles have long intrigued mathematicians.

1. The puzzle is complex and difficult to solve.
   The puzzle has a certain number of empty spaces.
2. The puzzle cannot be solved except by a computer.

The term PSPACE-complete is a measure of mathematical complexity and difficulty.

1. It's a puzzle no one has ever solved.
   It's the best puzzle for a child.
2. It's the most difficult simple sliding puzzle.

Quzzle's inventor claims it is the hardest simple sliding puzzle to solve.

1. 1880s
   1960s
2. 1980s

Rubik's Cube was all the rage during the early 1980s. A total of more than 350 million have been sold since the Cube was introduced.

1. 5 minutes
   2.5 minutes
2. 25 seconds

Fischer solved the puzzle in 25 seconds on the Tonight Show.

1. 21.71 seconds
   15.66 seconds
2. 5.66 seconds

At a 2011 contest, Latvian puzzle whiz Feliks Zemdegs unscrambled the cube in 5.66 seconds to set a world's record.

1. Feliks Zemdegs
   Nob Yoshigahara
2. Junk Kato

Nob Yoshigahara, who died in 2004, was one of the world's leading puzzle designers. The Nob Yoshigahara Puzzle Design Competition is held annually.