Mathematics.
8 + 9 = 7 + 10 (we moved a one to the nine to make a 10, we use 10 frames in class to help students visualise this) = 17.
If you are practicing this at home, please make sure that students show their working! Or you could just ask them how they arrived at the answer. Next week we will be looking at breaking double digit numbers into 10s and ones for addition.