There's a big tank of denture gum guarded by nurse Milly.
She won't let you have it so you have to get rid of her, first.
This is the low-spoiler guide for "Denture gum" puzzle in The Darkside Detective - A Fumble in the Dark. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.