Why does a peson sometimes experience a "shock" when touching the metal side of a trampoline?

Static shock is caused by an electrical charge built up in a person, changing them from being neutral to being negatively or positively charged ... The material of the trampoline surface is usually the culprit; if the surface is made of nylon it a good conductor for electricity and can cause static shock.

When you touch a doorknob (or something else made of metal), which has a positive charge with few electrons, the extra electrons want to jump from you to the knob ... Static electricity happens more often during the colder seasons because the air is drier, and it's easier to build up electrons on the skin's surface.