Opposites
Two-and-a-half is the most unpredictable preschool age. He will shift from one extreme to the other without warning. This is not because he purposely wants to confuse you or irritate you or defy you. The reason for this changeability lies in a giant step he is taking in decision making.
He is discovering opposites and alternate choices. He understands yes and no, run and stop, take and give, come and go, etc. But now he cannot sort out the relative merits of these opposites for any particular situation. He is not yet capable of thinking of only one to the exclusion of the other.