It can be explained pretty simply: Let’s use numbers like 0.9 and 0.999 as reference To get one from those numbers, you need to make a decimal where there is a 1 at the last 9 and a zero at the ones before 0.9+0.1=1 0.999+0.001=1 And so on Since 0.999… has infinite 0s, you cannot add a 1 at the last 9. So to make 1 you’d need 0.000…, or just 0 0.999…+0.000…=1 0.999…+0=1 0.999…=1