I keep my own knives pretty sharp. A bit obsessively so. But I don't always carry sharpening tools with me. Being used to sharp knives makes blunt ones almost unbearable. So, when I am helping a friend in their kitchen, and they ask me to cut something up, I need a way to sharpen a knife just with what I have around. Good thing my Oma taught me one that only needs a second knife, and I have learned another one using a mug.
Step 1: Using a Second Knife
This one is actually kind of nasty. Over time it will leave knives looking nasty and you will need to get them re-ground, but in the mean time it makes it possible to cut up those tomatos.
To sharpen a knife with a second knife (which gets sharpened as well in the process), grind the edges against each other as shown in the video.
You will probably end up with an uneven edge, but it will be sharper than before.
Step 2: Using a Ceramic Mug
Most ceramic mugs have a ring around the bottom where they are not glazed. The material that sticks out is hard and rough enough to rub off metal. Rub your knife over it like in the video and you will see the ceramic get dark where the metal rubs off. Again, rub the knife over the ceramic at an angle just slightly steeper than the edge it had before so you need to remove only a little bit of metal to get a better edge than you had.
Certainly not a way to prepare a scalpel, but you can really improve a blunt knife on it a few times before you can get it properly sharpened.
Step 3: Is It Sharper Now?
Have another look at the edge of the knife after you try each of these two options. If you did it right the shiny spots on the edge of your blade should be gone. If they are smaller but still there you might need to sharpen some more. If they have not changed at all, try changing the angle of the blade.
Another test would be to wash off the knife and try cutting a tomato. If you can get through the skin (the tomato's, not yours) without mangling it your knife should be sharp enough for most kitchen tasks.