We purchased some of these Contigo bottles for DD to take to preschool, she takes one every day.

When she brings them home I empty them and rinse them and pull off the straw (that is still attached to the top), then DH washes and puts in dishwasher..... I was happy with them UNTIL I noticed mold is forming where the straw attaches to the top... there is a small soft plastic piece and there is mold there!!! What the heck.. what are we doing wrong.. I feel like every single sippy cup/thermos/ you name it gets gross and moldy