The worst advice I ever gave when teaching new DMs, and I've since figured it out, is to tell them to say "yes" to everything.

Players generally don't know what they really want, it sounds mean but it's true. A player comes to you and asks if they can spend hours of their down time crafting items so they can sell them for money. The idea is "the player wants funds to buy something", however what the player really wants is something unique to set them apart, like that Longsword of the Vampire he saw someone have so he's trying to find a way to do it without asking you straight up for the weapon.

Players want to feel epic and feel like they're more powerful than anything else in the world. When they feel like they're losing that status they'll pull out stuff that makes them feel that way, they'll try to cut corners or manipulate the rules to their favor. It's not malicious, it's simply trying to fulfill that desire. There are the outliers, the ones who are trying to game the system, cheat, or otherwise just want to be the star all the time. They're outliers, most people aren't like that.

I could go on, but those two examples are best and they're easy to resolve: "Yes, but...". I now live by the actions have consequences philosophy, and it makes the game so much smoother. You can, as was said by Mystral, make DC checks that reduce the players' chances at success due to the frivolity or absurdity of the idea. You can create results that impact the choice negatively, such as the disappearing acid. You can also make it so that the idea is balanced and give the player a choice to take it or leave it.

A player wanted to create custom beast shapes with unique traits for their druid. His druid had a fascination with reptiles and would only beast shape into lizards. He sat down one night and went to work, unknown to me, he then handed me a stack of beast shape ideas that he'd come up with. I told him I'd look into the shapes and see what I thought, yes but... I then took and compared each shape to what was available at his level and adjusted them as I felt needed. It took close to a week of this before we came to an agreement, and some of the shapes were simply thrown out or restricted to higher levels.

Once you start using "yes, but..." consistently your players will begin to understand that they should put thought into what they're asking. It will give them the freedom to explore and allow you to reign in the shenanigans.