python kivy button and triggering a function

i m relatively new to kivy and i wanted to make a app but i m stuck at how to call a function by clicking on a button but i seem to face many problems .After hours of googling i still couldn t come up with a working code . i want to display the button first and then on click the button disappears and the function is executed.

import kivy
from import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout   
from kivy.uix.textinput import TextInput  
from kivy.uix.button import Button
from kivy.uix.widget import Widget

class maingame(GridLayout):
  def __init__(self, **kwargs):
    self.cols = 1
    self.rows = 1 
  def potato(self):
    self.add_widget(Label(text="potato is great")

class myApp(App):

  def build(self):
    cake = maingame()
    btn = Button(text="start " ,size=(90,90))
    return maingame()


if __name__ == "__main__":