"Fragile"

Alex had just stormed away. The Doctor considered it a pretty safe bet that she'd convinced the TARDIS to hide her room until the boys thought better of what they'd said. Specifically Jack.

"I didn't mean to offend her," Jack insisted.

"Consider who you're talking about," replied the Doctor. "Alex resents the idea that she might not be good or strong enough. Did you honestly thing she'd take well to being called fragile?"

"It's a relative thing," defended Jack. "She understands relativity!"

"But she detests being reminded of her weaknesses."

Jack sighed. "Next to you and me, how can she not seem fragile, Doc? She's only human. She doesn't heal as fast as we do, or as well. And if she dies, she's not coming back."

"I understand, Jack. Believe me, I do," the Doctor said, "but this is Allie. Fragile though she may be by our standards, compared to most humans she's pretty tough." He crossed the room and sat down next to Jack. "I know as well as you do that people who aren't like you or me are eventually going to leave or die on us. But because of that, not despite it, we have to let them be a little reckless. They've only got one shot. We can't treat them – including her– like they're made of glass."

"You did hear what she was suggesting, right?" replied Jack.

The Doctor nodded.

"Doc, there is a difference between reckless and deadly. Her idea kind of crossed that line. By a lot."

"Jack, there is a difference between worried and blindly overprotective. You're straying dangerously close to the latter."

"Point taken."

The Doctor had been half-right about Alex's hiding spot. She'd been damn near impossible to find, but she wasn't in her room. He found her asleep in what she had dubbed his "nostalgia closet" – the large room in which he kept clothes he'd worn in previous incarnations – wrapped up in his 4th self's scarf.

"Allie?" he said tentatively, kneeling next to her and touching her shoulder.

"No, I'm mad at you," she mumbled. "You have to call me Alex."

"Why are you upset with me? I haven't done anything!"

"You agree with him."

The Doctor sighed. "For a moment, would you stop acting like he murdered your puppy – to borrow your phrase for it – and get a little bit of perspective?"

Alex shot him an irked look, but didn't say anything.

Taking this as an invitation to keep talking, the Doctor continued, "Jack is very old, by human standards. I'm several hundred years older than he is. We've both got a long time ahead of us and an escape route from death. Next to that, how could a regular human not seem... fragile?"

Alex flinched. "I know. I understand, sort of. It's just... my brothers always kind of treated me like a responsibility when we were younger – like I couldn't deal with anything on my own, just because I was smaller and not as strong and sometimes I can't see. But you two know better than anyone that I can handle myself. So if even you see me as breakable..."

The Doctor put his arms around her. "He didn't mean it like that, Alex."

"I know," Alex said after a short pause. "It just kind of touched a nerve, you know?"

The Doctor nodded. "I know."

"I apologized," Jack told the Doctor later.

The Doctor smiled. "Good. Now don't do it again."

"I won't," replied Jack, sounding rather like a little kid who'd been caught doing something he oughtn't to.

Which, arguably, he was.

"Fragile" is a relative term.

Compared to glass, plastic is pretty strong.

Compared to diamond, it's very easily shattered.

Likewise, next to an immortal and a Time Lord, Alex – with her one short life – seems incredibly breakable.

But next to most humans, she is so very, very strong.