While in the Jungle (2-1 only) or Ice Caves (3-1 only), you will see a pink sticky blob on the wall. This is the Worm's tongue. If you stick a Damsel to it — whether she is alive or not — the Worm will come out a few seconds later and will swallow her. As long as you hold the damsel, the worm will NOT appear. If you are stuck to the tongue as the damsel is being swallowed, you will go in with her and enter the Worm. The easiest way to do this is to jump onto the tongue while holding the damsel, release her, and then wait in place.