Marcus heard about square roots, that they were tricky and hard. He wanted in. So I taught him a trick - if you know that a number is a square (16, 121, 5041), then it is not impossible to work out the square root in your head. Squares only end in 1, 4, 5, 6, 9, or 0. If it ends in 1, the square root ends in 1 or 9. If it ends in 4, the square root ends in 2 or 8. If it ends in 5, the square root ends in 5. If it ends in 6, the square root ends in 4 or 6. If it ends in 9, the square root ends in 3. And if it ends in 0, the square root ends in 0. He memorised this list more or less instantly.
So, if I say to him 5041, he knows it ends in 1 or 9. He can do 60 x 60, 70 x 70, 80 x 80 pretty easily, so he can find a range to look in. 5041 is between 4900 and 6400, so the root must be between 70 and 80. Its pretty close to 4900, so chances are that out of 71 or 79, the answer will be 71.
It takes me about five minutes (while driving) to work out a question for him. It takes him about 25 seconds to work out the answer. It only works with perfect squares, so it is not a particularly useful formula in daily life, but its a neat number trick.