how can usui's চুম্বন on ayuzawa's back "while she was wearing a bikini" leave a mark ???

i watched the জীবন্ত and read the জাপানি কমিকস মাঙ্গা for about 8 times in total but still cant understand,,,,i know i am stupid but just till me: how can usui's চুম্বন on ayuzawa's back "while she was wearing a bikini" leave a mark ???
জীবন্ত : episode 16 -or 17-
জাপানি কমিকস মাঙ্গা : chapter 18 -or 19-