"Why did you try to put the cat in the box...?"

"two minutes after I told you not to?"

He says, "I don't know."
What he means is, "I'm only five. I have a short memory and poor impulse control. I don't quite understand the consequences of my actions and the potential for physical harm either to me or the cat. I thought it would be interesting to find out what would happen, and, despite your admonition and my brief compliance to cease and desist, I went ahead with my original course of action."

Of course, if he could articulate all of that, he probably wouldn't be putting the cat in the cardboard box.