[It's a perfectly chaste and friendly kiss, but nice enough that she believes he definitely has kissed someone before. Which puts an amused half-smile on her face, as she steps back enough they can each have their personal space and not feel awkward.]

There's a few explanations for that. One's rooted in Norse mythology. The other comes from druids in...1 AD, I think. At least I think that's the year it was when the Doctor brought me there for a visit.

[She has to think on that, nose wrinkling up as she can't quite remember.]

Doesn't matter. Point is, mistletoe was the only plant that used to blossom in the winter. So they sort of viewed it as a fertility thing.

[Her eyes glance back up toward it, her smile a little more warm and genuine as she feels in her element teaching someone something.]

But I think people today just enjoy finding any reason they can to kiss and be happy.