Ki Ryn said:
Or if you want to confuse your DM, cast Silence on a flask of holy water (in your possession) and then throw it at your vicitm, breaking the flask and dousing him with the silence-bearing liquid. It probably won't work, but it should bring the game to a halt long enough for a quick cigarette break.
An interesting puzzle, and one that I hadn't heard of before (nor the breaking-apart an object with
light on it).
However, I think it's pretty efficiently solved by noting that (a) a body of liquid isn't an object, and (b) stuff inside or concealed by a magically obscured object is concealed only "effectively", not by there needing to be magic on it. (DMG p. 79: "An invisible creature can pick up a small item and hide it on his person... and render it effectively invisible.")
Hence, cast
silence or
invisibility on a flask of water, and the water is undetectable, but apparent as soon as you pour it out again. Similarly, an invisible chest reveals its gold once it's opened up (unless the gold has been independently treated).
The breaking-apart of a lit object is not so easily solved, but I'd propose a "biggest remaining part gets the magic" solution, similar to how I believe regenerating creatures are supposed to function (DMG p. 81: "Severed parts die if they are not reattached."). This would be an additional, supporting argument for an enchanted flask of water spilling out the water in individual particles, and only the flask (or biggest shard of it left) retaining the magic.
If you allow casting a spell on a block of ice, then I would in fact suggest that once the ice melts, the object is entirely disposed of and nothing exists for magic to still be targeted on.