You were going to go for something else, but once you saw the sword in your hands, you thought of an interesting hypothesis.
You took the longsword and planted it on Michelle's empty waist where her legs were.
"You gonna cut me up more?" Michelle asked.
"No, not yet. Just wanted to test something," you reply.
You look over to Michelle's legs, which were still standing where you left them, and their owner follows your gaze. You both see a very different pair of legs than what you remembered.
Instead of feet, the legs now stood on the tips of sharp blades which extended in width as they went up until they were about as wide as Michelle's calves were before at the knees. Speaking of knees, they were now mechanical looking hinge joints. They were attached to leather calves that, aside from their texture and brown color, still looked the same as Michelle's thighs. That was until you saw their connection to Michelle's waist. Hidden beneath her 'skin' you could see more mechanical joints connecting her legs to her hips, although they looked more like ball-socket joints, allowing for just as much movement as before, perhaps even more. Her hips still looked the same. Under her skirt, Michelle's ass still looked as fleshy and full as before. You could've checked further, but without her permission, you had to assume that the other side was still the same too.
"Woah! What'd you do to my legs?" Michelle said with more awe than shock.
"Well you see..." you explained the sword and its abilities. "...and that's when I thought: Hey, if it applies the traits of anything, what if I could give the traits of the longsword to you? Cool, right?"
"How'd you know it would work and that it wasn't a prank?" Michelle asked.
"...magic?"
Michelle felt a twinge of anger, but decided to set it aside. "Whatever, can you put me back on my legs?"
"Oh, sure thing." You lift your girlfriend's upper half back onto her lower half.
"Ah, that's better."
"So how do they feel?"
"Hmm. Dunno." She flexes her sword legs and taps the tips of their blades on the floor. "About the same, I guess. They feel pretty natural."
"That's good. I was worried you'd have trouble standing on your new...swords." You avoided saying feet.
She lifted one up to see for herself, impressively standing on one thin blade. "Yeah. Even I'm impressed I can stand on these things." She flicked it with her finger. "So...my legs now have the sword's magic power, right?" she asked setting her sword foot back down.
"Uh... I think so. They should anyway. I think," you say very confidently.
"Hmm. Well there's only one way to find out," Michelle declares before...