When it comes to "what gives it value" I usually take the Socratic Method here, and try to put things into perspective by asking what gives any other form of currency its value. If they say money has the backing of the government, bring up how that might not always be the case as with Zimbabwe's hyperinflation. If they say bitcoin has no intrinsic value and they raise gold as an example, highlight how any industrial application of gold is minute compared to what value it serves just because of its scarcity.