from superturtle.animation import animate
from turtle import *
from turtle import forward, right


begin_fill()

def square(side_length):
    fillcolor('blue')
    begin_fill()
    for side in range(4):
        forward(side_length)
        right(90)



for frame in animate(40, loop=True):
    size = frame.interpolate(50, 100, mirror=True)
    square(size)

    end_fill()