What is a 21 day challenge?

I don't know what the 21 day challenge is either. I checked out your Aug challenge SanDiegoJohn and your sites. Good challenge, but I would have to make some changes for me to do it. I can't afford to go to a gym, so the work outs would have to be some thing I could do for free at home or running around my neighborhood. I also can not completely give up fast food, but I do try to limit it and choose healthier choices. My boyfriend is the one that does the cooking for the family most of the time, and some times we are busy with his son's after school sports activities and so we some times end up going that route. The rest of the family doesn't want to change to a healthier menu or life style either, so there is always junk food in the house. But I have pretty much gave it up and I make sure to keep lots of veggies, salads, fruits and other stuff for me to munch on.