DD is 20 months, first child so I have no idea what I'm doing really so please be gentle!
Typical encounter; end of lunch. She's eaten quite a lot but is starting to chuck the rest around. I say "Are you finished?", she says yes so I take the plate. She points at the plate and wails. "Do you want some more?" "yes" Put the plate back, she shoves it away and turns away as if it's the worst thing she's ever seen. I take plate, she cries, wants it back, shoves it away. Repeat until I lose the will to live (usually about 3 times).
It's the same with a lot of toys, playdo etc. Do they just not really know what they want a lot of the time? She's not got many words so can't express herself very well, hence a lot of wailing which is part of what really drives me up the wall.