Anyone that watches SciFi knows that the big downfall to holograms is that you can't touch them. You just pass right through them, ruining the illusion. However, in real life there are several projects working on creating touchable holograms. The problem with these projects are - of course - the cost and availability of tools. But the general idea is that you use focused sound waves to create resistance at various points in mid-air to mimic the feel of virtual objects. New work is going into creating cheaper ways to do this, including printing tools that help shape sound waves in 3-D printers. Of course, this only works for static objects, but its still a new idea that will get more sophisticated over time. The video above really doesn't make much sense outside of the article, but it is pretty interesting at the end where they make drops of water levitate in mid-air.