Fair is signed exactly like equal: bend both flat hands in front of you and make the fingertips touch a couple of times.
Teach your toddler the fair sign when he or she throws a tantrum and wants to take more than their fair share of something from their sibling or friend. This is an abstract concept, so you can show gradual examples of how people are 'equal' regardless of gender, race, age, and so forth. "Tommy gets two ice cream scoops if you get two ice cream scoops. That's fair."
You can teach the concept of 'fair' in the sense of equality or fairness at play, taking turns, sharing, and so on.
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