[He's not sure what he expected - of all people, Keith knows the second he turned this into a challenge between them that it'd go in this direction, even as a joke. Maybe it's exactly because he knows, that his brain took it in this direction before he could act better on it. And Keith is far from weak, but suddenly it feels so so easy to turn into jello.]
[Still, because he wouldn't have a good excuse otherwise he matches him in spark, not serious either but quickly throwing an arm around Shiro's shoulders for leverage so he can try to jump, so that Shiro scoops up the air and not him at first.]