两个轮子的自行车大家再熟悉不过了。如果多给你一个轮胎，你可能会嫌它累赘、多余。就像一对情侣在约会时，旁边还有一个朋友作陪的`话，这个朋友就会显得有点不合时宜了。
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指不发挥任何作用的人和事物，或是在两人约会时陪伴在旁的一个多余的人，即我们所说的“电灯泡”。之所以叫third wheel是因为这样的人就跟自行车上放第三个轮子一样，一无是处。
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.
赖瑞和詹妮昨天叫我跟他们一起出去吃饭，但是我没去，我可不想当电灯泡。
