They can mellow out your harsh cowboy coffee
Eggshells usually go straight into the trash or the compost bin, but, like garlic skins and potato peels, you don't necessarily have to throw them out. They have a lot of calcium, which means they can be useful for gardening. If you're really deft about breaking your eggs, you could use a shell as a planter. And if you really want to, um, I guess you can eat them as a calcium supplement? I cannot endorse this. No eggshell I have ever accidentally eaten part of has made me feel like I should eat more of them. But you could. You sure could.
But really, if you want to give eggshells a second life, the trick is to apply them to your coffee game. Really! It sounds weird but it absolutely works. Crumbled eggshells are often added to cowboy coffee. When you boil the grounds with the bits of shell and water together and then strain the whole thing it helps tame an overly acidic or strong cup. But if you're someone who tends to overbrew your coffee, or if you're in a situation where you have some unfamiliar equipment and find that the coffee you're making is closer to sludge and/or jet fuel, eggshells will save you. Add crumbled eggshells to the grounds before you brew it, and you'll get a mellower cup.
The other way that eggshells can be useful is in cleaning up coffee. Do you have a mug with permanent coffee stain at the bottom? Or a charming carafe gone dingy? The calcium in eggshells can be super helpful for that! Just crumble some eggshells into the cup, add some water, let it sit overnight, and voila—the stain should be dissolved. Similarly you can use them as an abrasive cleaner for your water bottle. Crumble eggshells into the bottle, add water, and shake it around to clean up any build up. See? Eggshells can be your friend, and not just totally ruin your scrambled eggs.