For the past year I've used an electric toothbrush and I've always thought that the little light on it is light-sensitive. When you turn off the bathroom light it knows there's nobody in the bathroom and it automatically turns off its charging light. Handy, right? It's like a little reverse night light. Then today I set up a fan in my hallway, where the nearest power outlet is in the bathroom, and I discovered that the fan powered off when I turned off the bathroom light switch. Duh! I can't believe I never knew this. In two seconds, so many mysteries were solved! This is the reason that my razor is sometimes curiously uncharged when I leave it charging over night. And that other electric razor that I got frustrated with and threw out because its battery refused to charge, it was never given a fair chance.

(from a washington post article's comments)
not worth the finger strength I used to click on this article