It would also be impossible for the bug to read a book by the pool, since once the pages were scaled down to bug-size, surface adhesion would keep the pages stuck together. Small size and the small influence of gravity are also factors for flies on the wall, not to mention the millions of other insects, lizards, frogs, geckos and such that can climb walls and windows. Many walls have textures so they do not appear smooth, but window glass appears smooth to us at our level of size and scale. The segments, or tarsi, at the end of insect legs possess claw-like structures that help the insect hold on to apparently smooth surfaces. These tarsal claws are used to grip the tiny irregularities on surfaces that are actually rough at their small scale.
Geckos have a somewhat similar strategy.
The high-surface area foot produces plenty of places to grab a wall surface. One consequence is that smaller animals are more influenced by surface forces such as adhesion. This results in larger animals being more influenced by gravity. If a gecko on the wall were to drink Alice in Wonderland's growth potion and isometrically grow in size by a factor of 10, its volume and its weight would increase by a factor of about 125, whereas its surface in contact with the glass would increase by a factor of about 100. The poor gecko would fall from the wall because the somewhat increased adhesive forces could no longer resist the greatly increased pull of gravity.