Good ab workouts at home?
Im a 16 year old guy, I am looking to build ab muscle at home. Im skinny so i dont want to lose weight or burn fat, i just want to build ab muscle. Ive done simple crunches before but they dont seem to work very well and when i kept increasing the amount every week, i was up to about 200 crunches a day, and that took too long and i felt like i could do crunches forever, they werent challenging enough. So what are some good ab muscle building workouts that i can do at home that dont take very long? Im doing them at home and i dont have weights so dont suggest that. And id like info such as how often i should do them and when to increase. Thanks!