Throughout elementary school, students learn about positive numbers and how they fit on a number line. The transition to using negative numbers, after only working with numbers greater than zero for several years, can be difficult for students (Murray 1985). The card game described here was used successfully with fifth graders; through the game, students explore the characteristics of numbers less than zero and how to add positive and negative numbers.
Dickman, K. & Bofferding, L. (2016). Zero Pairs: Learning about Additive Inverses. Mathematics Teaching in the Middle School, 22(5), 308-314.