ダン, ノン, あたた.か, あたた.かい, あたた.まる, あたた.める
JLPT1 G6 S13 F1371
Two once-separate characters combine 耎# (soft) + 火 fire or 日 sun/day, respectively → warm up and soften (by fire or by sunlight; compare the right-hand element of 捏) → *warm/heat up*. The present form, which has assumed this meaning from the others mentioned, combines 日 sun/day + 爰.
[Full Kanji Etymology List]