Let play a game : I toss a coin, and each time it get Head, I earn 10 dollars, and each time it get Tail, I loose 10 dollars. My teachers says :
"I stop the game the first time I get 100 dollars" is a stopping time since at each launch, I know If I earned 100 dollars or not, but "I stop the game the second time I get 100 dollars" is not a stopping time.
I don't understand why it's not a stopping time, because I also know at each launch if I already got twice 100 euros or not. So could someone give me more details ?