Strike that.

Skin.

More specifically the skin of Chase's back and the way it seemed to glow golden in the light of House's bedside lamp. The way his back went up and down as he breathed softly. The soft skin covered by his shirt during the day.

Or was it skin, meaning the soft rough hot cold feeling of Chase against him? The palms of hands, the insides of arms, the shrug of a shoulder?

Or was it much more simple? The way the skin of his face curled up into a smile?

House would have to think about this.