Can you go on crash diets BUT keep the weight off after?

If you keep the weight off after being on any fad diet, then you'll be rich & famous before the week is up. I don't know about the whole soup idea, but eating healthy & exercising is what will keep the weight off. "Crash" diets are called that because your system crashes after being on it a while. A weeks worth of soup isn't going to hurt, I wouldn't think, but if you only ate low cal soup & got no other vitamins or minerals, & was on it for a month or longer, that's a crash diet (to me) & that would hurt your system.