Most people are still confused by the 'decentralized' nature of bitcoin, so in order for them to be more curious, I'll just say that there isn't a central authority issuing bitcoins, unlike the money that they are using today. So, this would be what I'll tell them: Bitcoin is digital money, but unlike your 'real' money, no central authority issues it, just a group of people with the hardware capabilities to 'mint' new bitcoins by solving complex puzzles. They'd be more interested on the "no central authority" phrase rather than "decentralized" trust me.