January 8, 2006: This is still sketchy, but I wanted to get it down: What I've been calling the "fluidity" of an interface is made up of 3 things:

• transparency (don't make me think)
• responsiveness (don't make me wait)
• "gestural efficiency" (How efficiently I can complete common multi-gesture acts)
  • distance betweeen gestures (literal and cognitive)
  • necessary precision of each gesture
  • interruptions

Finally, a perfectly fluid interface would feel more like dancing than working, which suggests that rhythm and balance play a role as well.