Many children learn to say "no" before they learn to say "yes". As an incentive to get my youngest daughter to say "yes" I would ask her if she wanted some ice cream. If she did not say "yes" I would start to put it away. For several weeks the closest she could come to saying "yes" was to say "nnno" softly with a tragic look. But I then would ask her "Shall I put it away" Then the "no" was emphatic, and she got her ice cream.