Water as a "Harmful Acid" Apologies for the question which is far below the brilliance of the minds on this page. I have an A-Level in Organic Chemistry but I'm tying myself in knots with this one.

I'm a magician looking to create a Russian Roulette effect with water and acid (sort of!) the actual secret is that both solutions are water.

Is there a readily available metal; or substance that looks like metal, that would make the water behave like an acid to convince an onlooker that it is a potentially life threatening acid.

As a bonus question – is there a way to cheat the litmus test so that one solution (also water!) turns it red, the other reacting neutrally (that's the easy one!)?
If you are using water alone, then sodium lead alloy reacts as sodium would but, with the lead added, it's much safer.  But you can have a wider variety of options if, as the comments suggest, you use a dilute acid like vinegar (reacting that, for instance, with zinc).  Wear eye protection no matter what you do.