Methylcellulose is one of the most interesting modernist ingredients. It has the unusual property of gelling when it is heated and melting as it cools. One of the most dramatic uses of this is "instant noodles" when the diner has a squeeze bottle full of liquid that when squeezed into a soup instantly turns into noodles. It has also been used to make "hot ice cream" that melts as it cools.
In addition, it is often employed as a binder in coatings, such as fried chicken batter, because it will solidify as soon as it hits the oil, creating a barrier that keeps the oil out and the juices in. Methylcellulose can also be used to stabilize foams and emulsions.
Methylcellulose is made from cellulose pulp, which is taken from plants' cell walls. There are about 20 kinds of methylcellulose and while similar, they all have different properties. Two of the more common ones are Methocel F50. which is commonly used to stabilize foams and Methocel A4C, which gels at a lower temperature and is good in batters and coatings.
There are several places to purchase methylcellulose. We highly recommend ModernistPantry.com, they have great service and are really good to work with (because of this, we do have an affiliate relationship with them) and have a wide selection of methylcellulose. You can also find it at WillPowder and get larger quantities and bundles at ForTheGourmet.com .
Methylcellulose is typically dispersed in hot liquids, above the setting temperature of the type you are using. Some, like Methocel F50, can also be dispersed in cold water if using a blender.
Once the methylcellulose has been dispersed, you need to cool the liquid in order for it to hydrate. The hydration temperature varies for the different types but a good rule of thumb is below 15°C / 59°F. Most types need to stay at this temperature for about an hour. I typically let the liquid cool on the counter or in an ice bath and then refrigerate it for several hours to be on the safe side.