Some previous studies have suggested that challenging your brain with mentally stimulating activities might help.

VOA: standard.2010.06.17

We're still close enough to the summer that the air is not just filling us up with extra static electricity, so it's a little more challenging here.

我们现在离夏天更近了,这样空气中不总是,充满额外的静电，所以这时做实验,会有一些挑战性。

麻省理工公开课 - 化学原理课程节选

And I'd like to leave you with a challenging question.

下面我想留给你们,一个具有挑战性的问题。

麻省理工公开课 - 媒体、教育、市场课程节选

But the acquisition of radioactive material for a so-called 'dirty bomb,' which is radioactive material that is dispersed with a conventional explosive device, that is not that challenging.

VOA: standard.2009.05.01

We differ from lower animals and our brains, which are much more capable of storing and processing information, but we're living in a time of revolution when machines are challenging or competing with our brains.

我们不同于其他低等动物，我们的大脑更能够存储和处理信息，不过身处改革年代,机器开始挑战甚至是,和我们的大脑开始竞争。

耶鲁公开课 - 金融市场课程节选

It is a long-term endeavor and we both are united to fight against this endeavor, which is challenging our way of life and wants to challenge the way of life with the world."

VOA: standard.2009.05.13