Here’s how we’re making mental pictures of our doubles
2 + 2 = 4      Picture a wagon with 2 wheels on each side and 4 wheels total
3 + 3 = 6      Picture a 6 pack of soft drinks, with 3 cans on each side
4 + 4 = 8      Picture a spider with 4 legs on each side and 8 legs total
5 + 5 = 10   Picture 5 fingers on each hand and 10 fingers total
6 + 6 = 12   Picture a dozen of eggs with 6 eggs in each row and 12 eggs total
7 + 7 = 14   Picture two weeks on a calendar with 7 days in each week and 14 days total
8 + 8 = 16   Picture a 16 pack of crayons with 8 crayons in each row and 16 crayons total
9 + 9 = 18   Picture a semi (18 wheeler) with 9 wheels on each side and 18 wheels total
10 + 10 = 20  Picture all your fingers and toes for a total of 20

Here is also another doubles rap link we have used in class: 
Doubles Rap Song