What is the square root of 144?

Twelve times twelve is 144, so twelve is the square root. Square roots ask what number multiplied by itself gives the target, and the first dozen or so squares are worth knowing outright because they turn up everywhere in geometry and mental arithmetic: 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144. If you do not recall it, you can bracket the answer. The square is bigger than 121, which is 11 squared, and smaller than 169, which is 13 squared, so the root must be 12 if it is a whole number at all. The final digit is another clue, since a number ending in 4 can only be the square of something ending in 2 or 8. Strictly speaking 144 has two square roots, positive and negative twelve, because a negative times a negative is positive, but the square root symbol conventionally refers to the positive one.

144 is a dozen dozen, known as a gross, and it also happens to be a Fibonacci number.