At the end of the Being Animated tutorial I set some exercises – I hope you’ve done them by now. One of the questions I asked was why I chose a width of 21 pixels and not 20? There is no magic in the number 21 per se. Rather, the issue is whether or not the number is an even or an odd number. Harking back to our discussion about pixels you should realise [sic] that pixels on the screen are not like marks on a ruler. Marks on a ruler take up no space pixels, on the other hand, do. If you chose an even number of pixels for something you want to centre [sic] somewhere the centre pixel will be at the centre. Then you will have a different number of pixels on either side. To take an extreme example, if it was two pixels wide, then one pixel would be at the mouse’s location, and one would be left hanging. There would not be a way to balance it. However, if it was three pixels wide, one could be at the mouse position, and one could be on either side.