Python, but not programming, newbie here. I'm programming with lists and have run into an interesting problem.

```
width = 2
height = 2

# Traverse the board
def traverse(x, y):
    # The four possible directions
    squares = [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
    print squares

    # Remove impossible squares
    for square in squares:
        print "now accessing", square
        if (square[0] < 1 or square[0] > width or 
            square[1] < 1 or square[1] > height or
            square == (1, height)):
            squares.remove(square)
            print "removed", square
    print(squares)

# Testing traverse
traverse(1,1)

```

This results in the following output:

```
[(0, 1), (2, 1), (1, 0), (1, 2)]
now accessing (0, 1)
removed (0, 1)
now accessing (1, 0)
removed (1, 0)
[(2, 1), (1, 2)]

```

It completely skips the elements (2,1) and (1,2) -- without even checking them! I found answers here saying I shouldn't be modifying a list while traversing it, and yup, that definitely makes sense. Rookie mistake. But can someone tell me WHY it doesn't work? What's behind the veil of Python lists?