I've been feeding my GPO (presently about 2 ft. TL) live shrimp and crabs and finally decided to try the old jar trick. I put a coonstripe shrimp in a 7 liter plastic observation container that I'd gotten earlier from Acorn Naturalists that has a snap on lid, which takes a bit of effort to pop open. I'd just fed the octopus another coonstripe about twenty minutes earlier, so I wasn't sure that it would even be interested. The container lid had trapped a bit of air in the container so it floated around in the tank for 5-10. The octopus eyed it a few times and touched the container rather tentatively without seeming interest. Then all of a sudden, wham, it grabbed the container, straddled it, evidently popped open the lid by sucking on with its suckers, and then checked out the shrimp with one of its arms. After that the action was hidden from view by the octopus's webbing. I had other duties to attend to but when I retrieved the container an hour later, no shrimp and the lid had been wrenched out of its hinges, so the suction must have been pretty strong. The largest suckers on the animal are about 1/2 inch in diameter and it grips very well when I clean the tank and it "wrestles" me.
That was the first time I had put any food in any container, but it didn't require any learning time at all - just wham, bam, thank you ma'am. Pretty amazing.