The following doesn't contain spoilers. The first half of this special is kinda of meh compared to the best they've presented us in the show, but it was still funny. But the second half... it was simply the best riddle of the series, I was left completely dumbfounded at the end by the brilliance of it. It really should've been part of the season in my opinion. So, if you've enjoyed this series, or really just enjoy riddles and/or science in general, this special is absolutely mandatory! A 9/10 for me. Disclaimer: If you figure it out before they reveal the answer, it might not be asenjoyable for you as it was for me.