@Apoorva, stack being "fixed" is not really true. In contrast to heap you cannot allocate memory on the heap freely, i.e. as you like. Nevertheless, stack is dynamic, too.
Example: recursive functions. For every call to a function, the local memory required for this function is allocated. If the depth of recursion depends on user input, the amount of memory required is not known when the program is started.
Therefore, the difference between stack and heap is not "dynamic" or "fixed", it's how freely / self-determined you can allocate memory on it.
