So what if aliens bleed acid? We breathe acid, suckers! And we can prove it! And while we're doing that, we can show why soda is very mildly bad for you.
The infamous tooth experiment which is meant to demonstrate that a tooth put in soda will dissolve within a few days, is now known to be a myth. A tooth doesn't dissolve in soda. If it did drinking citrus juice or eating salad with vinaigrette would dissolve teeth much quicker, as they contain stronger acids.
But sodas do contain acid, not just as flavoring, but as a result of the same thing that gives them their fizz. Carbon dioxide is dissolved in soda until it's supersaturated. When a can is opened, a lot of the carbon dioxide bubbles up again, but not all of it. Some of it combines with water. The CO2 and H2O forming H2CO3 - carbonic acid. This is the same process that helps form acid rain. Too much atmospheric carbon dioxide dissolves into the water in the atmosphere and makes rain into a weak acid.
And this weak acid is something you can make yourself, with some water and a straw. (It will help if you also put a liquid pH indicator in the water. Buy one or just puree some red cabbage leaves in water.) Grab the straw and blow bubbles into the water like you're a kid at a boring restaurant who wants to annoy its parents. Within a few seconds, the pH indicator in the water should begin to change color. You're making the water more and more acidic. Colder water can hold more CO2 than hot water, and putting a little pressure on the water can help it hold even more CO2. Get some pH strips and see how acidic you can make it with just your breath.
It's not the kind of acid that's going to burn through metal, but whenever I rewatch Alien, I like to think of the creatures talking together. "You know that stuff that we have to keep locked up in special passageways in our body? The meat creatures exude a vapor that turns water into that stuff. Badass."
[Via Steve Spangler Science.]