兩個輪子的自行車大家再熟悉不過了。如果多給你一個輪胎，你可能會嫌它累贅、多餘。就像一對情侶在約會時，旁邊還有一個朋友作陪的`話，這個朋友就會顯得有點不合時宜了。
A third wheel is a person or thing that serves no useful purpose, or an unwanted third party accompanying two people on a date. Such a person is as useful as a third wheel on a bicycle.
Third wheel指不發揮任何作用的人和事物，或是在兩人約會時陪伴在旁的一個多餘的人，即我們所說的“電燈泡”。之所以叫third wheel是因爲這樣的人就跟自行車上放第三個輪子一樣，一無是處。
For example:
Larry and Jenny asked me to go out for dinner yesterday, but I said no, because I didn’t want to be a third wheel.
賴瑞和詹妮昨天叫我跟他們一起出去吃飯，但是我沒去，我可不想當電燈泡。
