Rounding to the nearest ten means replacing a number with the closest multiple of 10. For example, 47 rounds to 50 because 47 is closer to 50 than to 40. The rule: look at the ones digit — if it is 5 or more, round up; if it is 4 or fewer, round down. Numbers ending in exactly 5 round up by convention.
Open number lines showing two neighboring tens with the target number placed between them make rounding visual. Students mark the number and identify which ten it is closer to. Avoid introducing the rule before the concept.
Rounding is about choosing the best approximate value for a number when you do not need or want an exact answer. Every whole number lives between two neighboring multiples of ten. The number 47, for example, sits between 40 and 50. Rounding to the nearest ten means picking whichever of those two neighbors is closer. Because 47 is 7 away from 40 but only 3 away from 50, it is closer to 50 — so it rounds to 50. Your prerequisite knowledge of the number line makes this visual: place 47 on a number line with 40 on the left and 50 on the right, and you can see it is nearer the right.
The ones digit is your shortcut for making that distance judgment without actually measuring. The ones digit tells you how far you have traveled *past* the lower ten. A ones digit of 0, 1, 2, 3, or 4 means you are less than halfway to the next ten — so the lower ten is closer, and you round down. A ones digit of 5, 6, 7, 8, or 9 means you are at least halfway — so the upper ten is closer (or equally close), and you round up. The rule "5 and above round up" is not arbitrary; it follows directly from measuring distance.
Place value — your main prerequisite — is what the rounding result is built from. When you round 47 to 50, you are changing the ones digit to 0 and increasing the tens digit by 1. When you round 43 to 40, you are changing the ones digit to 0 and keeping the tens digit the same. The tens digit is the only digit that matters in the result; all smaller digits become zero. This is exactly what "rounding to the nearest ten" means: you are replacing a precise number with the closest number that has a zero in the ones place.
One edge case deserves attention: what happens when rounding causes the tens digit to change in an unexpected way? Consider 95. The ones digit is 5, so you round up. But up from 95 is not 100 — wait, actually it is. 95 rounded up to the nearest ten is 100, not 90. The tens digit becomes 10, which carries over into a new hundreds digit. This surprises students, but the number line confirms it: 95 is exactly halfway between 90 and 100, and by convention we choose the upper neighbor. The same carry mechanism you know from addition applies here too.