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.
Caviar is just small spheres normally made with the direct spherification process. If reverse spherification was attempted, as the caviar was dropped into the setting bath they would stick together on contact.
You can find more information about caviar from my guide on how to make caviar or from the caviar articles below.