This is going to be my last post for a couple of weeks as I'm off to do some travelling. Optical Illusions & Visual Phenomena by Michael Back is an interesting website full of some really trippy optical illusions. Guaranteed to make your head spin in next to no time.
If you look around in the below figure you will notice the appearance and disappearance of black dots at the crossings.
Way back in the mists of time before the Internet existed I grew up on a cute little personal computer about the size of a paperback book with rubber keys. It was the beloved Sinclair ZX Spectrum, it had a whole 48K of memory and you would load in software from a cassette tape (assuming you were lucky enough for the game to load without the dreaded tape loading error). I spent hours on this computer doing all sorts of weird and wonderful things with it, but I'll have to admit I did enjoy playing games on, especially one of my favourite games of all time.
I suppose you could describe it as a super mario type game where you had to collect lots of discarded glasses and bottles from various rooms in a mansion. The original version and a spectrum emulator you can download from worldofspectrum.org. I've recently discovered a PC port of Jet Set Willy which is pretty fab, which you can download from retrospec.
There are lots of other classic games on this site that have been ported to the PC, such as Manic Miner.....the prequel to Jet Set Willy.