I'd just make it simple and go on to explain that bitcoin serves as the equivalent of fiat currency for online transactions. But, the people who buy and sell it around the world determine its worth. The government is not able to exert control, unlike fiat. I'll tell them it can grow value and reduce value with time due to some certain factors. And that they can invest and if they are lucky enough it might give them double of their investment but then they should only invest money that they can afford to lose.