My 16-month-old GSD growls when I try to take his ball away. He'll bring it to me to throw but then won't give it back. I don't want this to escalate. How do I

Don't take the ball away — trade for it. He's guarding because you've been using force to end the game. From his perspective, you're a resource thief. Here's the new protocol: when he brings the ball back, show him a second ball or a high-value treat. The moment he drops the first ball to take the new item, pick up the first ball and throw it. He gets to keep playing and you get the ball back. Repeat. You're teaching that giving up the resource makes good things happen, not bad. After 20-30 reps over a week, introduce a verbal cue: say 'drop' right as he releases the ball to grab the treat. Eventually he'll drop on cue without needing to see the trade item first. If he's already escalated to snapping, stop playing fetch entirely for two weeks and work on the trade protocol with lower-value items first. Build the skill, then reintroduce the ball. Never punish the growl. It's a warning. If you suppress it, he'll skip straight to biting.