When I take Isaac to the park to run, I've been doing something new with him just recently. He has to sit while I take off his leash and wait for the OK from me to take off running. This is completely doable for him. He has to wait like that for permission to start eating when I feed him. But this is new for him and while he is happy to sit when told, it's hard to wait when he really really really wants to run.
A couple days ago was the first time I did this with him at the park and it was so funny. I have a hand signal I use with Isaac that means stay. I hold my hand up, palm out, like I am saying "stop." So I had my hand about three inches in front of his nose and told him "stay" and took off his leash.
Well, he started to take off running. And plowed right into my hand. And came to a stop because how could he keep running with my hand smashed against his nose? And he looked up at me like, "What the _______?" He just seemed surprised and confused that my hand was in his way.
It was hilarious.