It is widely known that Isaac Newton invented the cat-flap. As the great man sat grappling with some of the most complex thoughts in history, he was occasionally disturbed by having to put the cat out. Newton turned his enormous brain to the problem, and burned a hole in the door, so he could get back to redrawing the universe. These days, you can get cat-flaps for doors,windows and outer walls, which all come with a template to draw around. On doors, drill holes at points around your template and cut out the shape with a jigsaw, or, for the non-electrically inclined, use a padsaw (about £10), which cuts curves in wood. Be careful not to make the hole bigger than the flap; sounds obvious, but it happens. The two halves of the cat-flap then simply clip together and screw into place. With glass, it's 90% certain you'll break the pane if you try it yourself, but there's no harm in trying - either way, you'll end up at the glaziers. Score around your template in one motion with a glass-cutter, without overlapping on the join, then score crosshatches inside. Tap around the shape until the crack goes all the way through, then repeat on the crosshatches. It'll never work. For walls, hammer-drill your holes, then chip out the brick with a 'cold chisel', a heavy finger of metal about an inch wide. Wall cat-flaps are not always lined, so line the tunnel with plywood. My dad uses lino (which the cat now prefers to her basket). Owners of bullied cats should go electromagnetic, so the flap opens only for a special cat collar. It is satisfying to hear Tiddles hurtle into the kitchen, followed by the thud of his bully against the shut flap, learning that action and reaction are equal, and opposite.