One + One = Two… except for large values of one. That math joke refers to rounding off numbers. When rounding off to whole numbers- we usually round to the nearest value. So 1.45 rounds off to 1 while 1.55 rounds off to 2. So 1.45 + 1.45 = 2.90 which itself rounds off to 3- unless you round off early and incorrectly. Then 1.45 rounds off to 1 and you get 1 + 1 = (in this case) 3. I didn't say it was a great math joke- but it helps illustrate the trouble we can get into with numbers.