One of the most attention grabbing modernist techniques is spherification. It's the process of trapping a liquid inside of a gelled sphere. The gelling agents used in spherification only gel in the presence of certain ions, such as calcium or potassium.
There are two main spherification techniques, reverse spherification and direct spherification, depending on where you put the gelling ingredient. If you add the ingredient to the setting bath it is "reverse spherification" and if you add it to the flavored liquid, it is "direct spherification".
Sodium alginate is a good gelling agent to have on hand because it is very effective at both reverse and direct spherification. Once set, it also can be heated above the boiling point without melting, making it very versatile.
I find reverse spherification easier to use than direct spherification because you can freeze the liquid into spheres before gelling it and because the spheres stop gelling when you remove them from the setting bath.
You can get more information about reverse spherification from my guide on how to make reverse spherification or from the reverse spherification articles below.