Helium. It's colorless, odorless, tasteless, non-toxic, inert and useful in everything from better hard drives to laser fusion power. Given these incredibly handy properties and helium's finite supply on Earth, scientists really wish we'd all stop wasting it in balloons already.
Helium may be common out in the solar system, but around here, it's a lot harder to come by. We have to extract it from natural gas, and when it's gone, it's gone. No more. At all. Ever. At current usage rates, some scientists put this point at anywhere from 25 to 50 years from now, where if we don't exactly run out of helium, it'll become scarce enough to have severe consequences for important things that depend on helium to work, such as MRI scanners.
This is why some scientists are calling for helium to be a.) much more expensive than it currently is and b.) regulated like any other precious commodity. With regards to a.), helium is cheap right now because the U.S. stockpiled a bunch of it starting in 1925, but it's all in the process of being sold off. Scientists who may or may not be being slightly melodramatic estimate that it should cost about $100 to fill your average party balloon. In the words of one frustrated researcher, "when you see that we're literally just letting it float into the air, and then out into space inside those helium balloons, it's just hugely frustrating. It is absolutely the wrong use of helium."
The upshot of making helium more expensive is that people like those at NASA would stop wasting as much of it as they do. For example, as of 1996 NASA was using approximately 20 to 30 percent of the country's entire yearly helium supply, pressurizing and purging rocket engines, and then just letting it escape into the atmosphere. NASA could have recovered it and reused it, but helium is so cheap that the agency just didn't bother. The Air Force does the same thing. So the point of making helium more expensive would be more to encourage conservation than to deprive children of balloons.
Speaking of balloons, there's a simple alternative to using helium: you can just fill party balloons with hydrogen instead. Yeah, it explodes sometimes, but guess what, things explode sometimes. It's a valuable lesson, kids. Learn it and live it.
So from now on, whenever you see a kid with a balloon, you will be expected to grab it from them and stomp on it while screaming "YOU'RE KILLING SCIENCE!!!" Thank you for your service.