Mind the Gap

In London, the underground railway system is called the "tube." As you walk down the steps to the platform to wait for your train, you hear a voice on the loudspeaker calling out the approaching trains by repeating the same three words: "Mind the gap." Right before you step off the platform onto your train, you see those same words just behind a thick bright yellow line. Why is this message so important that it needs to be constantly repeated? It's because if you don't mind the gap, you could end up falling into the space between the platform and train. This area is highly electrified, so when it happens few have lived to tell the tale.