For as long as he's been talking about it -- since last winter, maybe -- Ulysses has used the word "dancing" to mean all kinds of music. If someone is playing an instrument, or singing, or dancing, he gets happy and points and says, "Hey! Dancing! Dancing!" Usually then he imitates whatever the person or puppet or cartoon character is doing.
If he's upset, and I try to soothe him with singing, he gets angry and says, "No! No dancing!"
This morning, he said to me, for the first time, "Sing!" Tonight he was watching Finding Nemo (his new favorite show since last weekend -- 11 million viewings and counting) when I came home from work. He sang along to a number where the characters were singing in low voices like some sort of Tiki gods, and turned to me: "Sing!" He pulled my jaw down so that my mouth opened. "Sing!" he commanded until I began to sing along to the song I had never heard before (I haven't watched the whole movie yet, myself) as best I could. That pleased him. "Singing!" he said, and smiled.