I counted the clicks made by my turn signal, and when I got home I used a clock with a second hand to determine how long it took to click the same number of times. Other things I could have done was to time my windshield wipers (assuming it was raining) and to time how long between certain points in a song on the radio (assuming I was listening to the radio, only one song played while I waited, and the song was not overly repetitive).

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