Coolis signed differently depending on your meaning. If you want to say it's cool outside, not necessarily cold, take your dominant hand close to your face and making a fanning, up-and-down pivoting action with the wrist of your open number 5 hand.
If you mean that something is 'cool', as in fashionable or interesting, take your open number 5 hand and stick its thumb end to your chest while wiggling the rest of the fingers as the palm faces the non-dominant side of your body.
Teach your baby or toddler the cool sign in the context of cool weather, or when you are describing something as 'cool'.
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