I got my current cat when she was about 8 weeks old and clearly malnourished. I brought her home, fed her, ran out to buy more supplies, and when I got back she sat her little butt down, stared up at me and went:
Eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
... for about 30 seconds straight. This was to let me know more food was required. She still does it, but usually can't sustain it as long as that first time.
(Siamese mutt and very mouthy.)