Slug Mucus

Snails and slugs can traverse vertical and inverted terrain because they crawl on slimy gel that is a complex fluid with non-Newtonian properties. In the most basic terms, the slime acts like a solid glue at rest, but liquifies when an adequate stress is a applied (a stress exceeding the apparent yield stress). When the stress is removed, the slime quickly re-solidifies. By exploiting this yield-heal property, a snail can keep part of its foot stuck to the wall, while other parts move forward. The picture shows the bottom of a Leopard Slug, Limax maximus, during locomotion.

Here is a video of the experimental setup ((987 KB) (Randy Ewoldt).