Depending on your social media foot print you may have seen a lot of this image over the last few days:
If you didn't achieve "genius" status the first (or second) time you looked at it, that's ok. It is a bit tricky and I don't think solving this has a direct correlation to your IQ. To be honest, I missed the closed pipes the first time I looked at it too. Some people got really upset with others over this thing. Massive debates were sparked and it was determined "Well, we just can't be sure, without doing it." Hands on experimentation to the rescue!
A facebook page called Prozix made a 3D print of this now infamous brain teaser and did the actual experiment. Now you can confidently tell your students the answer when they try to stump you. Video of the experiment is available here (or here) but make sure you try to make your own guess before viewing.