Thomas spent most of his time lurking under and behind rocks. Some predators like Thomas find this a useful default activity, since you might be thinking about nothing at all when, Oho! prey wanders by, and because you are under the rock, they suspect nothing.
When Thomas detected my sister heading for the refrigerator he truly shone as a wily predator. If she was going for “a glass of milk and a salami sandwich” Thomas took no action. But if her intent was to take out tubifex worms for the eel, Thomas was somehow able to tell. He swam out of the rocks and wriggled back and forth enthusiastically just below the surface. (It is not clear to me whether fire eels salivate, but if they do, Thomas may have been drooling.)
We can't figure out how Thomas knew when my sister was after human food and when she was after fish food. This was not a matter of timing, since Thomas was fed irregularly, at any hour of the day, and not on all days. It will probably remain a mystery, since it is no longer possible to do experiments (on my sister, not on the fire eel) under the same conditions. (Was she walking differently? Was she wearing tubifex gloves? Was she glancing at the tank? She says not.)
Perhaps you wonder why Thomas was not offered salami. Salami is fatty and salty, and probably not particularly good for little fire eels. Unlike many fire eels, Thomas was a good neighbor who did not eat the other denizens of the aquarium. On his salami-free regimen he waxed mighty, getting to be over a foot long before he perished in a reckless expedition to travel outside the aquarium. He was found in a desiccated condition on the keyboard of a nearby computer.
Let that be a lesson to all of us.