what's the best way to get frozen mice to be warm enough after thawing them?

I've been putting them in the small bag they come in in a cup of really warm water from tap. Not boiling or anything but definitely hot to the touch, and leaving it in there until it's all not frozen anymore, usually he will take it on first try, however this time while putting it into the enclosure i accidently dropped it in the water bowl, probably making it no longer warm from the water, which is why he didn't eat it. Luckily the pet store is right next to my college, and I have class in about an hour, so I'll pick up some bigger foods and try again today.

thanks for all of the help everyone =)