add an image to GridLayout in a .kv file from python

Question

How can I add an image object to a GridLayout defined in a .kv file?
Is this possible? I have tried ui.py

class Field(Widget):    
    grid = ObjectProperty()
    
    grid.add_widget(Image(source="res/Water.png"))

class UI(App):
    def build(self):
        return Field()

ui.kv

<Field>
    GridLayout:
        cols: 9
        id: grid

I get when I run ui.py

 Traceback (most recent call last):
   File "ui.py", line 7, in
   File "E:\Scripts\Python\kivy\ui.py", line 8, in <module>
     class Field(Widget):
   File "E:\Scripts\Python\kivy\ui.py", line 17, in Field
     grid.add_Widget(Image(source="res/Water.png"))
 AttributeError: 'kivy.properties.ObjectProperty' object has no attribute 'add_Widget'