求翻译：今天，我和几个同学出去，我们准备去步行街买衣服，我们遇到了一件有趣的事，我们在公共汽车，我们看见一对兄弟在对话，他们差不多只有4、5岁。一个小孩说，爸爸是奶奶生的，那奶奶是谁生的呢？另一个小孩说，我知道，奶奶生爸爸，那爸爸就生了奶奶。我们大笑了一场。是什么意思？

Today, I and several schoolmates exit, we prepare to walk the street to buy clothes, we have experienced an interesting matter, we in the bus, we see pair of brothers to converse, they almost only then 4, 5 years old.A child said that, the daddy is paternal grandmother fresh, who is that paternal grandmother lives? Another child said that, I knew, paternal grandmother lives the daddy, that daddy has lived paternal grandmother.We have laughed.