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Eight word pairs, each shown together so the contrast does the teaching. Opposites are worth more than sixteen separate words: learning big alongside small gives a child a way to compare things, which is the start of describing anything at all.
A pair carries a relationship a single word does not. Big only means something against small, and children grasp the contrast faster than either word alone.
Around four to six. Concrete pairs such as big and small or hot and cold come first; more abstract ones like open and closed follow.