"<method> takes no arguments (1 given)" but I gave none

I am new to Python and I have written this simple script:

#!/usr/bin/python3
import sys

class Hello:
    def printHello():
        print('Hello!')

def main():
    helloObject = Hello()
    helloObject.printHello()   # Here is the error

if __name__ == '__main__':
    main()

When I run it (./hello.py) I get the following error message:

Traceback (most recent call last):
  File "./hello.py", line 13, in <module>
    main()
  File "./hello.py", line 10, in main
    helloObject.printHello()
TypeError: printHello() takes no arguments (1 given)

Why does Python think I gave printHello() an argument while I clearly did not? What have I done wrong?