How does a fly land on the ceiling?

Flies are quite cunning. They have little grappling hooks on the end of their feet, and when they're going towards a surface, they literally have to throw themselves straight upwards towards the ceiling so their head points towards the thing they want to hit. They reach out with their front legs and they grab hold of something spiky or rough that they can latch these miniature grappling hooks onto. They then switch off and fold their wings away, and swing their body under them in order to lock onto the ceiling. Now they're hanging on upside down. When they want to drop away again, gravity does the work for them. They can just let themselves go, but they hang on with some feet first so that one part of their body drops away and then the rest of their body drops after it so they're in the right position to begin flying again. People know this because they wondered that very question and someone clever invented a camera that could take pictures fast enough, called time lapse photography. You can get pictures now at 4000 times a second so you can see individual wing beats of insects and things. Now they were able to work out exactly how they're doing this.