How does this thermometer work?

i have this kid bathtub that had a thermometer inside, it's not digital or electric, it's round. my kid was playing with it and took it apart (so my husband tells me, i wasn't around). so today i took a look at it to check it out, and to see if there was any mercury, since i was worried about it, but there didn't seem to be any? or anywhere to keep it anyway. there is just a round space with a paper with the degrees (that i threw out) and in the middle a little black thing where the hand was that indicated the degree.

i am still a bit worried, should i be? we keep the tub in the regular bathtub so it gets washed out all the time.

but i'm also wondering how it works otherwise. i have no clue if it was accurate, i never paid attention to it, although the hand would move around.