When you add two numbers that add to more than 9. So I you were to add 7+9 you get 16. The 6 would fall under the ones' place value and the 1 would fall in the tens' place value. Another example: 24+16, the 6+4 gives you 10 which is more than 9. So take the zero and put that in the ones' place. Then carry over the 1 into the tens' place. So now instead of just 2+1, you have 2+1+1. This will equal 4 and fall into the tens' place to ultimately give you 40.