After trying to beat marathon mode (finish 200 lines) in Tetris DS for a couple days, I finally did it before History class today.
After unlocking the endless mode, I took it a step further...
I love the feeling when I make mistakes and block up 3/4 of the screen and work my way back down towards the bottom.
That probably happened like 5 times during that game above and I was about to give up, but I kept on going. Lesson of the day: never quit.