It can be anything, from teaching them the name of one ball from another, (Big ball, rope ball, etc.) to shaking hands...if they don't already know it.
Their minds are engaged, they like the interaction, and seem pleased with themselves when they are praised, or treated, for learning something new.
Now, to teach Charlie his numbers, so that he can bring you your next In Death book in the series when you re-read them! ;-)
Love All Life, Thank You For Posting! :)