Hyundai

If it's anything like my Kona the cap can be turned a small distance then you have to press down and turn it some more to take it off. Probably designed to stopp people getting scalded when the take the cap off whilst the vehicle is hot.

Chris
Most caps have a small amount of turn to release pressure to the overflow tank, before actual removal. There is a stepped ramp system of filler neck and cap to make this happen. Simply to protect the person from hot/scalding coolant.
DOES yours do this? If not you may get a dose of hot flushes.

Good to see it's just a radiator cap our 20011 elantra done the gear box last week on holidays 125,000km had to leave it at hervey bay continued on to Bris in a hire car $6,500. Bloody trip tronic auto's unlike my GQ' patrols manual that was replaced a few months ago at 500,000km