Okay, maybe he misjudged about what Kaylee would think of his level of sanity. He says earnestly, "I've not told ye this before because...well, to be honest, I thought ye would think me mad." And he's really hoping she doesn't think so now. He's not sure he could deal with that.

But as he's thinking about how he can convince her, he realizes - he knows Ten's sonic screwdriver wasn't working properly. And he knows that Kaylee, being Kaylee, was probably very happy to help take a look at it.

"Ye saw the sonic screwdriver, aye? Is there technology like that on the ship?"