To sign elbow, simply bend your non-dominant arm and form a fist with that hand. Then take your dominant hand and use its index finger to point to your non-dominant arm's elbow.
Teach your child the elbow sign in context: when you are teaching the body parts to your toddler, or when you are instructing them to straighten their elbows up to get their shirt off, and so on.
Teach your toddler this sign along with the other body parts when they are ready. It is good to complement the body parts flash cards with nursery rhymes, like "Head, Shoulders, Knees and Toes".
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