Loretta's mind went off at a tangent .

洛蕾塔的思想突然离题了。

One moment the professor is working hard on a problem in physics, the next he’s gone off at a tangent and he’s talking about bees.

前一分钟教授还在努力解答一道物理题，下一分钟他就突然开始讲起蜜蜂来。