Oh no, it's a rock!

For the last few months Dominic has had this little ritual with me, and apparently only with me. I have no idea why we do it, but we do it every time I go to unbuckle him from his car seat. Every time he will be curled up in his seat and I have to say "Oh no, it's a rock!" before he will uncurl and let me unbuckle him. I have no idea what this is about. :-)