That way they all feel like I've given them the time of day, and no one feels neglected. Take for example the universal: 'Uh-huh'. But 'Hmhm' also deserves a mention! The more universal the answer, the better.
One might argue that 'Yes Anwers' are pretty universal too, but these are not without danger. Before you know, it you've given your kids permission to polish off fifty cookies, or to take a vacation by themselves in Ibiza. When this happens you see'll a triumphant gleam in your kids' eyes, and immediate measures should be taken.
Unfortunately this means you'll have to stop using universal answers, and focus all your attention on the child in question. Then you have to tell him, or her, that the universal meaning of 'Yes' is sometimes 'No'.
This is a universal problem for moms, for which there is unfortunately, no universal answer.