Using the Pen
Put together a block of commands like this

This will repeat the following set of commands 4 times:
move the turtles forward 10
steps, turn left 90 degrees
The turtles will have walked out a
square, but it can happen so quickly that you don't always see
it.
Turtles carry a pen, and by
commanding them to put the pen down before moving means that
they leave a trail, enabling you to see where they have been.
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