What appears to be a smooth surface, like a ceiling, for example, is actually covered in tiny cracks and bumps too tiny to be seen by the naked eye. These can provide insects like flies with a sort of toehold, or rather, a hairhold.
A fly’s legs end in clawed segments called tarsi, which have pulvilli, or large footpads, at the bottom. The pulvilli are covered in lots of setae, stiff bristle-like hairs. The setae produce a glue-like goo that lets the fly adhere easily to smooth surfaces, but also enables them to quickly take off.