I teach using the word GENTLE. The dog should take it out of your open palm gently, so you start there, and when you offer the treat in the open palm, if he takes it gently, say GOOD GENTLE.
Start reminding him when he takes, GENTLE. Always tell him Good Gentle if he takes it nice.
When that is good, start putting a thumb over top of it and make it a little harder. Remind him GENTLE and praise with the word. Do not let him have the treat if he does not take it nice. After he is doing good with this.
Close the fist and only open it if he is licking and gentle. If he is being rough then, keep it closed, put it back in your pocket. But if he takes it gentle use your words.
Now make it even tougher, and remind him before you offer up your thumb and forefinger to his fangs. Gentle, then offer the treat between your thumb and forefinger. This seems to be the obvious way people offer treats -- especially strangers who will definitely hold it against the dog if the dog takes finger along with the treat. So it is definitely worth it to teach the dog to take it gently even when held precariously like this. The thing is, you will have built up to this.
Once the dog has learned to take the thing out of your hand nicely every time, no matter how you are holding it, you can expand the command, by saying, "GENTLE with my fingers" or GENTLE with Grandma. Remind the dog as he is coming in, and you will see him put on the breaks and be more careful.
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