Both "גולגולת" (skull) and "גלגל" (wheel) are listed, on Wiktionary, as coming from the shared root ג־ל־ג־ל. All of the other words except for גולגולת have clear relationships to rotating (to roll, cylinder, wheel, etc.). There's also the word גָּלִיל, "cylinder", and Wiktionary lists that as being "related" -- but I'm curious what that means, is it just superficially similar? Did they influence each other? Or do they derive from a shared stem? Thanks.