How to gnaw? Of course with your mouth and teeth. 口(mouth) + 齿(teeth) → 啮(gnaw) niè
What do you wanna keep in your mouth? Of course something delicious or sweet (甘).
What do you feel if you taste something sweet with your tongue? Of course you feel sweet. 舌(tongue) + 甘(sweet/sth. sweet) → 甜(sweet) tián