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

VOA: standard.2010.06.17

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

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