Spiders build their webs to trap flies and other insects for food.  An insect is unable to escape once it has become caught in the spider’s web.  The more the insect struggles, the more it becomes entangled in the sticky threads. A spider’s silk is strong enough that most insects cannot break through it.

A web-spinning spider does not become caught in its own web.  When walking across the web, it grasps the silk threads with special hooked claw on each foot.  The spider also secretes an oily liquid onto its legs and feet that prevent the sticky silk from sticking to its body. – Dick Rogers