You skip a meal, or decide that a scoop of cottage cheese is a wiser choice for lunch that a double beef bozoburger with bacon, guacamole, and cheese.
Before long, the energy-distributing molecules in your bloodstream start to become scarce. Your body starts slowing down to adjust to the situation. You may feel cold, since less energy is available to be burned. Your stomach starts sending telegrams to central control, ``Hey, what happened to lunch?''
As the bloodstream becomes depleted in energy, the fat cells notice this and respond; now's the time to draw down the reserves. Perhaps the boss is stalking a mammoth and doesn't have time to scarf up some fruit and berries along the way (or maybe Monday Night Football's gone into overtime and the fridge is forgotten in the heat of the moment--the world of the fat cell is a simple one, hardly cognizant of such modern problems). Individual fat cells begin to tap their storehouses and release energy into the bloodstream to ameliorate the shortfall.
When this goes on, the rubber bag contracts: you lose weight.