In Python, you are given a function named outer_function() which has another function inner_function() nested inside it. The outer_function() has a variable number initialized with a value of 5, while inner_function() attempts to modify this variable.
However, there are some missing pieces in the code. Your task is to fill in the blank in the inner_function() in such a way that calling outer_function() returns the string "Number in outer function: 10".