I'm going through C# 101 class and I've encountered an error. So far we have put all our code in to the Main method and we started to break our code to methods. I'm having trouble since after running the program for the first, then modifying some code the the next time VS tells me that my program is used by another process

"Could not copy "obj\Debug\Homework.exe". Beginning retry 1 in 1000ms. The process cannot access the file 'bin\Debug\Homework.exe' because it is being used by another process'

How do I find out what is causing this and used by what process?