1+2=3, and ALSO 3-1=2 and 3-2=1. Sometimes 2+1=3 is also explicitly included. Multiplication/division families also exist, of course.

This is intended to strengthen the connection between addition and subtraction for those who don't just see it. I apparently comprehend backwards, since it is beyond obvious to me and always has been, and just look confused at them when they offer this to my daughters as an example of their new, deep approach to mathematical concepts.