"D is a Concentrate"

Alex gets off the bus most days singing some song he learned at school. Yesterday he wasn't singing, but showed me a note some girl on the bus had given him!  It had hearts all over it and his name.  It was good handwriting, so Bri and I think it's an older girl.  After I looked at the note and grilled him about it, he started singing, "D is a concentrate.  A letter of the alphabet."  I asked him if it was supposed to be D is a consonant.  He said probably, but he likes it better his way!