Why is it so????

As soon as we put a bite of food in our mouth, our weight will equal however much you used to weigh plus the weight of that bite. So after eating food, our new body weight equals our old weight plus the weight of the eaten food.

But this change is not entirely permanent. Your body processes the food and burns the calories to keep everything running smoothly. You'll soon excrete much of what's left of the food, and expel the moisture through sweating or urination. Body weight keeps constantly fluctuating throughout the day.