Total Product start to fall, if production continues by a producing firm in short run, even after Marginal Product becomes zero because of the following reasons: Better utilisation of the fixed factor: In the first phase, the supply of the fixed factor (say, land) is too large, whereas variable factors are too few. So, the fix factor is not fully utilised when variable factors are increased and combined with fixed factor, then fixed factor is better utilised and output increases at increasing rate. Increased efficiency of the variable factor: When variable factors are increased and combined with the fixed factor, then former is utilised in a more efficient manner. At the same time, there is greater cooperation and high degree of specialisation between different units of the variable factor. Indivisibility of the fixed factor: Generally, the fixed factor which are combined with variable factors are indivisible. Such factors cannot be divided into smaller units. Once an investment is made is an indivisible fixed factor, then addition of more and more units of the variable factor, improves the utilisation of fixed factor. The increasing returns apply as long as optimum level of combination between variable and fixed factor is achieved.