"What?" Sarah Jane said, mystified and indignant. She looked behind her and caught the Doctor looking away suddenly. He'd been staring at her rear! Him, the Doctor, not usual at all! The waistband of her underpants was showing.
But why was he giggling so much? They weren't even girly panties -- just some briefs from some American company, picked up at need. He'd certainly seen her underwear before. In fact, he'd seen her totally naked. You don't always get away from monsters with your clothes intact.
"Ah, I'm sorry," said the Doctor, still chuckling. "Gallifreyan schoolboy joke."