I was hiking and I saw a small lizard crawling up the side of a relatively smooth rock. I was amazed at how they can move so freely and not fall off. After a bit of a research I found out that their feet have hundreds of tiny hairs that help with sticking to surfaces. Also, their feet also act as little suction pads which undoubtably help with their wall climbing abilities. It would be an interesting project to see what kind of surfaces in which lizards can't scale. Some lizards climb better than others as well so it would be cool to have a wall climbing contest.
Flies and other insects are able to perform these gravity defying feats as well although they do so somewhat differently than lizards do. Obviously, insects are much lighter than humans and lizards so they require little effort for them to stick onto surfaces. Flies have little claws at the end of their legs which allow them to attach themselves onto a surface. At the end of the claws are sticky cushions but unlike lizards, they aren't like suction pads. These sticky cushions are just sticky enough to be able to hold the fly's weight which allows them to hand upside-down with ease.
I think it would be fascinating of humans could one day do as lizards and flies do and climb up vertical, smooth surfaces with ease. It would be best if everybody became like Spiderman but until that special radioactive spider comes along, it's not going to happen. I was watching an episode of Mythbusters and they were trying to scale a wall using vacuum-powered suction cups. They actually managed to climb up about 20 feet or so before one of the pads developed a leak and was no longer usable. Not only that but the pads were pretty large and could only be used with really smooth surfaces like glass. A rough surface would simply have too many air pockets that the cups couldn't form too.