I'm sure you guys already know, but they can also be taught how to open a closed jar if there is a tasty crab inside. First, just lightly put the lid on with a snack inside and he will lift it off. On day 2, barely screw on the lid and he will see that he has to slightly turn and lift. Then each day after that, slightly increase the tightness and soon the octopus will learn to wrap his arm around the lid and unscrew it in order to gain access to the meaty treat inside.
They are very intelligent creatures.