Nagaimo (長芋, ながいも) is a type of mountain yam used in Japanese cooking. While it may look similar to other tubers, grating it will result in a drastic difference! When grated, nagaimo turns into a very viscous, slimy substance that is commonly used as a binder in batters such as okonomiyaki. You can also cook it like a regular potato, and the gooey-ness will go away once cooked.
Care should be taken when handling raw nagaimo. Raw nagaimo can cause a slight allergic reaction to the skin, making it itchy, use a pair of gloves or plastic wrap when handling it.
Nagaimo can be found in Asian grocery stores. It is sometimes packed with sawdust to keep it fresh, so be sure to scrub it well before use!
You'll need to buy this at your local asian market.