building a yellow house

Noah Sirlin loves to climb to the top of the play structure.  He is able to do it carefully and swiftly and rarely needs help adjusting himself or getting down.  Today he said "Rebecca, I'm going to make you a yellow house."  He climbed to the top of his "ladder", hooked his underarms over the top rail and began moving his fist up and down and back and forth like a hammer.  What a great imagination...and... how did he know I've always wanted a yellow house!!!