17. It is forbidden, on Shabbos, to place water in the bran one is feeding to animals or fowl (1). Furthermore, if one placed water in the bran on Friday, one may not stir it on Shabbos (2). It is, however, permitted to pour it from one container to another in order to mix it together.
(1) One of the 39 Avos Melachos (prohibited categories of activity) is kneading ("Lisha"). Any action which involves binding together small particles (like flour), by means of a bonding agent (like water), to form one mass, might be prohibited on Shabbos, depending on the circumstances. Two common applications of this prohibition are mixing baby cereal with milk, or mixing eggs or tuna with oil.
(2) There are two steps in the kneading process: adding the bonding agent (which alone causes some of the particles to bond), and then stirring the mixture; each of these acts is prohibited. Now, in general, when a melacha is performed in an unusual manner, although it is not considered a Biblical violation, the act is still prohibited rabbinically. The melacha of kneading, however, is an exception to this rule, and hence, it is permitted when performed using halachically specified modifications to the usual kneading process, such as stirring in a "criss-cross" fashion ("Shesi Ve'Erev), rather than using a circular motion, or pouring from one vessel to another (see Shulchan Aruch 324:3).