This is a mural in Teck Ghee. I asked two uncles what it means. Argument ensues.
"It means we can’t think outside the box."
"No lah. It means that our brains are solid. OK? SOLID."
"It’s true I tell you. The man from the PA said so. So it is definitely true."
"You see? Talk like that - prove my point. I win already."