a straw that will change colour if a drink has been spiked

a straw can change colour.it hears seem pleasant . consumer will be fun and curious.straw are cheap but how can make it change colour .if we can ,we will have a lot of consumer because people like to see a new.we also need a factory to produce this product.don't talk more about this product, we must know how to make this product first....hehheeh