when you visit the popular website you tube. after looking at one video, you get distracted into watching an interesting looking video in the 'related videos' section. Another distraction follows, and another, and so on and so on, until you discover that you has spent a surprising amount of time, and are now watching something totally unrelated to what you went on there for.
Example 1: i've got about ten minutes to kill, so i think i'll hop on the you tube train and see where it takes me.
Example 2: you'd never believe where the you tube train just took me. i went on to do my favourite old search of 'fat kid falls', and i ended up watching a video of a dog dancing on it's hind legs to the james bond theme music. what an awesome world we live in.