When I was changing the baby that I babysit, she had black strings in her stool. What are they?

Question

Her mom said it was from the banana she fed her.

Answer 1

Yes, banana will do that every time.  It looks like little black worms.

Answer 2

Sounds like banana could be it but it may also be mucous if food has irritated her gut. If you see it in the next two stools, see someone (don't give her any more banana for 2 days to eliminate it)