Zevachim 21 - Kohanim Dipping Their Hands and Feet
The standard procedure for the kohanim to sanctify their hands and feet in the morning is as follows: the kohen is standing in front of the vessel, he places his hands on his feet, and the water is spilled on them from a spout in the vessel.
Could they to instead dip their hands and feet into the vessel? Can we derive the answer from another rule: if a kohen dipped his hands and feet in a mikvah, he still needs to sanctify them from the vessel. Can we conclude that it is only after a mikvah that he needs another sanctification? - Not necessarily, maybe dipping in the vessel is also ineffective.
Then why not state this rule about the vessel, and we will deduce the rule about the mikvah? Because you might have thought that, since one can ritually bathe the whole body in the mikvah, how much more so he is allowed to dip just hands and feet! Therefore, the rule tells you otherwise.
Art: Edgar Degas - Study of hands