同学之间不宜开的玩笑有那些？

One day I bought a pair of leather shoes in a shoes shop.
The seller there told me that he will have "three guarantees"
for the shoes. I left the shop. And only four days later the shoes 
were worn out, so I was angry and returned back to the shop
hope to get my money back. I told him what had happened 
to my shoes. When he knew I had bought the shoes for four days, 
he refused, that made me surprised. I asked him why he didn't
keep her promise. But the seller said," Do you understand the
'three guarantees'? It means that the shoes can only be guarantee
for three days!"
     What a foolish but sily explanation to  a "three guarantees"!