WHICH TANK FILLS FIRST? ft. Simone Giertz

Okay, hang on! There has to have been a last atom in before the first atom spilled right? And whether the last atom in went into 3 or 4, it went into 3 first... so 3 filled up first. Or did 3 only stop passing atoms to 4 when 4 was full making 4 the one that filled up first... except it didn't because the atoms are also spilling out of 3 and 4... so 3 filled up first! Wait, were they philips or flat headed screws?