You take a cross and a slide and put them together because you're a genius. Geniuses are always inventing new things, and your invention involves crossing this cross with a slide.
You use your incredible genius to create a cyborg (さい). It's shaped like a cross with a slide sticking out the back, but it's also got lots of human parts you stapled on there. You're a genius, so you don't have to explain why you made this cyborg though. No one can judge you! You did nothing wrong!