Okay, Nate officially understands the concept of numbers and letters. He doesn't know one from the other, but when he sees letters on a block he'll identify them as the number 2, or the letter D. Totally wrong every time, but at least he understands the concept.
He also understands 1 and 2, and sometimes 3. Last week he saw an umbrella with the same pattern as my umbrella, and said it was the "sames." And tonight he built a tower out of only blue blocks.