When Luke looks up for the tenth time in three minutes Sarah Jane sighs and tilts the screen of her laptop down slightly.

"OK, out with it," she says.

"Out with what?" Luke asks, looking around him for a clue. "It's not bin night."

"It's an expression," Sarah laughs. "It means tell me whatever is on your mind."

"Lots of things."

"So pick one."

"Which one?"

"Whichever one is... giving you the most concern. If there's a problem then I want to know about it. Maybe I can help."

"What is love?" Luke blurts out.

The question stuns Sarah for a moment as she tries to consider her response. He said 'love', not 'sex' so she doesn't think this is the "birds and bees" talk. Mind you, he's probably already aware of that thanks to Clyde.

"Love?" she asks.

"Yeah, love. What is it?"

"You don't ask easy questions, do you?" She smiles at the look of confusion that passes over his face. "I mean… It's really hard to define. Love is… well it's different for everyone."

"What is love to you?"

Sarah exhaled loudly. "It's... difficult."

"Why?"

"It's..." She stops and smiles. "For a teenager you are like a child sometimes."

"I am less than a year old," he points out – in all seriousness – and it's the straight face that makes Sarah's heart stop for a second.

Then she knows what she's going to say.

"Love is... amazing and terrifying and everything else. Love is wanting to be with someone, share your life with them, share their life. All the experiences that you're both going to have, and you want nothing but the best for them…"

She stops, realising that she's talking to a teenage boy. A hormonal teenage boy, who's going to school with lots of teenage girls and who is good friends with the lovely teenage girl across the road.

Maybe she needs to change tact.

"Love is... about wanting to make a life with someone. Spend your life with them." Her voice softens as she thinks about the men in her life who she could apply this to. "Sometimes love is what keeps you going through the bad times. People can hurt you and do wrong by you and you forgive them because you... love them.

"You'll follow someone and you'll stick by them even when it's hard. You want them to be happy and you'll do anything to make them happy and keep them happy." She laughs softly, the corners of her mouth curling up as she hears herself. "Sorry, probably sounding like a bad romance novel."

"It's fine," Luke says. "So love is... complicated."

"Very," Sarah replies. "People have been trying to work out what it is and why it affects us the way it does for… ever, really. Poets and authors and songwriters and no one can come up with an answer for what love is."

"So how do you know when you're in love?"

"I... You just do. I know it's a bit of a cop out but you... know. When you fall in love with someone, everything about them stirs everything in you. Your every instinct is geared around them."

"Right," he says, looking like he was processing all this new – and probably unhelpful – information.

"Why the question?" she asks, wondering if she actually wants to know the answer. If he's about to admit to liking someone she's calling for reinforcements and she very carefully reaches for her mobile and pulls up the address book. It's either going to be Clyde or Maria, depending on what Luke said next.

"Just... thinking about stuff."

Sarah gets to her feet, clutching her mobile like it's a lifeline, and moves over to him. "You can talk to me, you know? I know it's probably not the done thing, talking to your..." She hesitates over the word. It's still unfamiliar on her lips.

"Mum?" Luke finishes for her. There's a few seconds' silence before he asks, "Are you OK?"

"Why wouldn't I be?"

"You hesitated. Hesitation is a sign of nervousness or unsurety. Something is wrong. You are nervous about being my mother or you are unsure…"

"No," she interrupts. "Not unsure. Never unsure. Promise."

"Nervous then?"

"...I guess," she smiles. "This is all new to me."

"Me too."

"Just need to adjust to it," she tells him, rubbing her free hand down his arm.

"I think I've adjusted," he announced and it confuses her for a moment. "I've been trying to work out something."

"What?"

He smiles at her and she swears her heart literally stops for a second.

"I love you," Luke tells her and forget her heart – her world tilts before stopping. His face creases in confusion. "Is that the wrong thing to say? Have I made a social mistake?"

"No," Sarah whispers. "Far from it." She presses her lips together and nods once. "I love you too," she replies.

"I didn't understand about romantic love and parental love," he tells her, explaining his earlier question. "How you can feel this way about someone but you don't want..."

"OK!" Sarah interrupts. "I get it."

"It is OK, isn't it?"

It takes her a few seconds to get that he's asking her permission to love her, and her world lurches back out of kilter. "Of course it is," she whispers. "It's more than alright!"

He floods with relief and moves to hug her, and when her arms close tight around him he knows that he wants to spend forever with her and make her happy and keep her safe.

And she's thinking pretty much the same thing.