There are tons of ways to make a macro lens for your smartphone, but if you need one for a DSLR, it's not quite as simple as using a magnifying glass or a drop of water. If you have an old kit lens, though, you can turn it into a macro lens in no time—all you have to do is remove the front element.
For this hack, Juha Loukola over on PetaPixel used a Canon 38-76mm lens, but says that the process should be pretty much the same for other lenses.
First, he pried the plastic ring from the front of the lens with a flat-headed screwdriver.
Removing the ring exposes the screws that hold the front element in place. Now, all that's left to do is take out the screws and pull off the front element, and you have a makeshift macro lens.
It attaches to the camera the same way as normal, but auto-focus won't work without the front element. It can always be reattached anytime you need it, as well. Here are some of the shots that Juha took with his lens:
It's worth noting that this may not work for all lenses, but several commenters have said that it worked with theirs (including both Nikon and Olympus lenses). Another way to take macro photos without making any modifications is to use the reverse lens technique.
Check out Juha's tutorial for more details, and check out the comments to see what other lenses this trick works (and doesn't work) for.