Whatchamacallit

Have you ever had experiences like you know the object very well, but you totally don't know what it's called or you've totally forgotten the name? It happened to me at Belém Tower in Lisbon, Portugal. I was able to go inside of the fabulous World Heritage Site, and I saw this vehicle like object. Oops. There were a lot, but the name of that was on the tip of my tongue. Whatchamacallit? I don't know why, but I remembered the name of the ball; it's a cannonball. I don't know why I didn't remember the name of the whole thing. Now I know, it's a cannon. That simple, huh?