Why is the voltage source short circuited and current source open circuited when they are removed while solving problem on Superposition Theorem.

First of all we must know that the voltage source has an internal resistance connected in series with it and the value of that internal resistance must be negligible which is equivalent to short circuit…thus whenever we consider a single source, the voltage source is short circuited.
On the other hand, the internal resistance associated with current source is very much high, this amount of resistance minimizes the flow of current to a very less value such that we assume there is no current flowing in that branch….thus we consider it to be open circuited in case of single source assumption.