Answer:
how hot the spear actually is.
Step-by-step explanation:
In this simile, Homer is comparing something familiar, which is the way that an iron arm is made by molding it and then inserting it into a cold tub of water, with something unfamiliar to the reader (this is the basic functioning of the Homeric simile), which is how they attacked the cyclops with a spike that was heated so that, when it entered the Cyclops’ eyeball, the spear hissed just like hot iron does as it enters a cold tub of water.