My Mom will be watching our lo for 3 weeks when I go back to work. I pump and make bottles of milk for her. My Mom is adamant that the baby's milk be warmed up. I'm fine with that because she's pretty good about taking milk if it's warm, cold, room temperature, whatever. She's not picky.

Should I get a warmer for her to use or just have her warm up the bottles in a mug of hot water? If I should get a warmer, which one?