Why is that when i run the code sometimes . the word "nil" display?
So Ruby returns something at the end of your program. Have you noticed that when you do each exercise from the different Units? It could be any Object that was part of some expression the Ruby interpreted evaluated. In that case, if you get nil, that just means there were no expressions to evaluate ("null" or nothing), or your expressions just returned "null," which in Ruby is referred to as nil.