Disclaimer- sadly, I don't own Castle, Snow Patrol, , Bruno Mars, Guy Sebastian, Jordin Sparks or Taylor Swift.

A/N- this is my first Castle fic, no sure how I went, so let me know!

The Experiment

"How do you know you're in love?"

"All the songs make sense."

About an hour after Beckett left, Castle was sitting in his office, with a bit of a confused look on his face. All the songs make sense, that's what Beckett had said. Then Castle smiled; I might just have to test that theory.

He was going to need Alexis' help for this experiment, Castle decided. "Alexis!" He yelled up the stairs, "I need your help!"

Shortly after he had called her, Alexis came bounding down the stairs, "what with?" Alexis asked.

"I'm doing an experiment!" Castle said sounding a bit like a five year old, who just got told they could have the first slice of cake.

"At 10:30 at night?"

"Yes," Castle answered, his smile never leaving his face.

"On what?" Alexis asked.

"Whether the songs actually do make sense when you're in love, or if it's just a coincidence," Castle said trying to make it sound scientific.

"Sounds great, but, what do need me for?" Alexis said, usually she would love to help her Dad with an experiment. But it was 10:30, and she had school tomorrow.

"I need someone who's in love, and you're the only one I've got," Castle said.

"What about Gram?" Alexis said pointing to the room where Martha was staying with them that night.

"She's likely to give off a false positive," Castle said.

"What?" Alexis asked, not following her fathers reasoning.

"She's been 'in love' more times than I've been on the best-sellers list," Castle said with a smile.

"Dad," Alexis said, giving him that look that said, 'be nice, even if it might be the truth'. Then she said "fine, but then I'm going to bed, I've got school tomorrow."

"Are you sure you're not adopted?" Castle asked in mock seriousness.

"You know, sometimes I really do wonder," Alexis said in the same tone, then " so what do you need for this experiment of yours?"

"The songs that only make sense when you're in love, we'll play them, you listen to it, I listen to it and see if it makes sense to you and not me," Castle said.

"Okay," Alexis said doubtfully, I'll get my I pod."

Five minutes later Alexis had songs that she now understood properly, coming out of her I pod, the first of which was 'Art Of Love' By Guy Sebastian and Jordin Sparks.

I'm saying sorry in advance 'cause this won't always go to plan,
Though we don't mean to take our love for granted,
It's in our nature to forget what matters,
How when the going is getting tough,
And we're all about giving up,
Things that we never thought we'd gonna say, gonna say them,
Things that we never thought we'd play, gonna play them,
It ain't perfect, but it's worth it,
And it's always getting better,
It's gonna take some time to get it right…

As the lyrics played Castle tried to think about what it could mean. But he didn't have to try very hard because he had it with in the first verse. That song finished and he turned to Alexis, "well?" he asked, "what did you get out of that?"

Alexis answered him, "they love each other, but their relationship's not perfect so they're saying sorry," Alexis said.

"Huh," Castle responded thoughtfully, "that's the same as what I got."

"That say something to you Dad?" Alexis asked knowingly.

"Yeah, it does," Castle paused, "that was an easy song, what's the next one?" He asked, and Alexis sighed, and looked down at her I pod which was telling her that the next song was Chasing Cars by Snow Patrol, which she relayed on to her Dad.

"That's a harder song," Alexis said, as the chorus started to play.

We'll do it all, everything, on our own,
We don't need anything, or anyone,

If I lay here, if I just lay here,
Would you lie with me, and just forget the world,

I don't quite know how to say how I feel,
Those three words, are said too much, they're not enough,

If I lay here, if I just lay here,
Would you lie with me, and just forget the world,
Forget what we're told, before we get too old,
Show me a garden that's bursting into life…

"Okay," Castle said when he'd listened to the whole song.

"Ignoring the rest of the world, that's what you do when you're in love," Alexis said. Two from two, Castlethought.

"Yeah, that's what I thought," Castle mumbled.

"Next song?" Alexis said, wanting to go to sleep, sometime before she needed to wake up, but not before she got her point across.

"Yeah, next song," Castle agreed.

"The next song is Ours, by Taylor Swift," Alexis said, "I love Taylor Swift, Dad, you really got to get me tickets to see her in concert. Me and Ash have a song."

"Already?" Castle asked, disbelieving.

"Yeah," Alexis answered with an enthusiastic nod of her head, as the song came on and both Castles went quiet to listen to it.

Elevator buttons in morning air,
Strangers silence makes me wanna take the stairs,
If you were here we'd laugh about their vacant stares,
But right now my time is theirs,

Seems like there's always someone who disapproves,
They'll judge it like they know about me and you,
And the verdict comes from those with nothing else to do,
The jury's out but my choice is you,

So don't you worry your pretty little mind,
People throw rocks at things that shine,
And life makes love look hard,
The stakes are high, the water's rough,
But this love is ours…

The song finished and before he could ask any more questions about Ash, Alexis continued, "the next song is," she paused to look at the screen, "Nothin' On you by B.o.B and Bruno Mars."

Beautiful girls all over the world,
I could be chasing but my time would be wasted,
they got nothing on you baby,
nothing on you baby,
they might say hi and I might say hey,
but you shouldn't worry about what they say
'Cause they got nothing on you baby,
nothing on you baby…

This ones perfect for him and Beckett, Alexis thought as the chorus played, he's got to get it now. But sadly for Alexis, Castle must have chosen that night to be crazily thick, because he still didn't seem to get it.

"I'll go first this time," Castle said, "He can get any woman in the world, but he keeps chasing after one because he loves her and hardly even notices the other women…" Castle smiled, then his face slowly fell, "oh." Alexis could pretty much see the light bulb go off in his head.

Alexis?" Castle asked.

"Yeah Dad?" she asked.

"I'm in love," Castle said with a shocked look on his face.

"Welcome back to reality Dad, now I'm going to bed, night," Alexis said giving him a hug, and heading back up the stairs.

"Yeah, night" Castle said absently. Isn't it funny the things you learn from a midnight experiment? Castle thought to himself as he walked towards his own bedroom, still stunned.