After completing the challenge to make an empty square we can also make shapes but have them automatically fill in using the command begin_fill() and end_fill()

Before making the shape call begin_fill() and then once the shape has been done call end_fill()

For example, this would draw a triangle:

skk.begin_fill()
skk.forward(100)
skk.left(120)
skk.forward(100)
skk.left(120)
skk.forward(100)
skk.end_fill()

Try modifying the challenge you just completed so that the turtle now make a filled-in square