Most rounding is calm. The trouble starts when the digit you're rounding up is already a 9 — because 9 can't just "go up by one" and stay a single digit. It has to carry.
The Carry
Round 9.96 to 1 decimal place. The kept place is the tenths (the 9), the decider is 6, so round up. But the tenths digit is already 9 — bumping it up makes ten tenths, which is a whole one. So it rolls over: the tenths become 0 and the whole number 9 ticks up to 10. The answer is 10.0, not 9.10 or 10. It works exactly like carrying a column in addition — when a place fills up, the overflow moves left.
Keep the Trailing Zero
9.96 to 1 dp is 10.0 — and that final 0 is not optional. Writing just "10" claims you rounded to the nearest whole; the .0 is the proof you rounded to a tenth. Trailing zeros after the decimal point carry information: they show the level of accuracy. Drop them and you've quietly changed what your answer means.
Never Double-round
Press 2.349 in the toy. It's tempting to round to 2 dp first (2.349 → 2.35) and then to 1 dp (2.35 → 2.4). That's wrong. Always round once, straight from the original number. Rounding 2.349 to 1 dp: the decider is the very next digit, 4, which is less than 5, so it stays 2.3. Double-rounding let an early bump-up shove the answer the wrong way. One number in, one round, one answer out.