Every number has a neighbour just before it and just after it. One more means take a single step to the right on the number line; one less means a single step to the left. So one more than 7 is 8, and one less than 7 is 6.

This is the same hop you already use when you count — counting on is just saying "one more" again and again.

Something neat happens when you jump by ten instead of one. Because of place value, ten more and ten less leave the ones digit alone and only change the tens digit. Ten more than 34 is 44 — the 4 ones never moved, but the tens went from 3 to 4.

Press play: a marker lands on a number, then hops one step right (one more), one step left (one less), and finally a big jump of ten each way — watch the tens digit flip while the ones digit stays put. Replay it for a fresh starting number each time.

Khan Academy shows how adding ten changes only the tens digit here: