If your goal is to lose weight quickly, laxative, diuretics, and amputations will all quickly reduce your overall mass, at least as defined by what your read on a scale.
If you want to lose fat, however, you're going to need to pass most of it through your nose and mouth.
No worries, though, you'll first need to break down your globs of fat, rearrange them, and ultimately pass a few electrons onto oxygen to form water.
By the time you're done, most of your fat's mass is now carbon dioxide (the oxygen came from water during hydration reactions). The solid jiggly stuff that some folks see as a problem is released as a gas! But not from the end you might have thought....
(On the flip side, the solid stuff of trees is mostly from a gas--yep, carbon dioxide again!)
More fun with high school chemistry
ScienceQuest 2013