A square arrangement of dots grows by adding an L-shaped border each time: 1, then 1 + 3 = 4, then 4 + 5 = 9, then 9 + 7 = 16. How many dots must the NEXT border add, and what is the new total?

Natural numbers (1, 2, 3, …) answer 'how many?' when there is at least one of something. But what if there are none? You need a number for that — and that number is zero. Adding 0 to the natural numbers gives the whole numbers: 0, 1, 2, 3, …. Every whole number sits at a fixed point on the number line, and they never run out — for any whole number you pick, there is always a larger one.