fd moves the turtle forward. It is the opposite counterpart to bk.
The distance is measured in pixels.
fd 100
If a pen is selected, the turtle will draw a line as it moves.
pen red fd 200
If the turtle is turned, it will move in the direction it points.
rt 45 fd 75
fd 100 moves about an inch on a typical screen, because
most computers fit about 100 pixels in an inch. The graph paper
background is equivalent to a 1/4 inch grid: it has one square per
25 pixels.