I found out that U stands for (potential) energy in science, so like DrTom I was trying to use some sort of physics formula or whatever! :) (Even though that didn't seem to jive with how everyone mentioned what fun it was.)
Today, I returned for the nudges but didn't even need to open a single one -- Just had to see Ben's intro: "It's May 4th, so... "
Thanks and forces to you, Ben!