I am starting a seven day fast,or ten depending if i can get away with it, how c!

I am starting a seven day fast,or ten depending if i can get away with it, how can i keep this from my parents

I don't want to tell them because i'm afraid they'll disapprove....but how can i say no to food without them knowing somethings up? Also, please only helpful answers...thanks :) I want to do this fast to cleanse my system of toxins and such so that i can start a healthy diet.

The best way is to be open with your folks. Let them in on your plan. Be willing to discuss this as your "first choice", yet be open minded toward modifications and other options.
The two best things I can think of are:

1) do it on Dr's approval, advice, and monitoring
2) go by a written and medically approved plan, making sure you are very well hydrated through this time and this hot weather.

Seriously here, 7 days is an extra long time to fast. As your mom, I would probably object. Now, If it were a 1 or 2 day fast, there is less at stake in harming your health.

Remember that a "diet" = "a plan"
Total denial may not be the best route, out of experience I say this, because once you lose the sensation of hunger, It may not return to "normal" for years, and it can become a genuine health issue on the negative side (personal exerience here)

As you look over your options and choices, I hope you settle on what is the healthiest thing for your body.