Well, OP when such question is asked again I hope you will know something better to say than the above. bitcoin is not just a crypto currency it's a paradigm and the root to other cryptocurrencies that exist out there. so having to invest in bitcoin gives one the edge of confidence that their money is safe even though safety depends on your capacity to safe guard your wallet.
You should say something simple like bitcoin is far better than other cryptocurrencies because it's the king and can't disappear like other coins, other coins are shitty compared to bitcoin so you're friends can have a clue that bitcoin is a safe investment and a better currency.