A school in Ranchi must seat 1,000 students in buses that hold 45 students each. Since part of a bus cannot be used, how many buses are needed so that every student gets a seat?

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.