Just now, the boy John provided a valid clue, which Shard shared with the witch beside him. Audrey also believed that the missing two boys might have encountered that door which occasionally appeared by the river.
So the two continued to walk forward, and shortly after reaching the area the boy pointed out, Megan shared the Seven-Color Mushroom Potion with Shard, and they each dripped it into their eyes.
Because this was a diluted version of the potion, it certainly didn't have the same effect as directly consuming the mushroom. Therefore, they couldn't completely see the black cracks in the air; they could only see some twisted light and shadow, and because they couldn't thoroughly observe them, they couldn't touch them.
Continuing to walk forward, carefully observing the area by the river where the door might appear, Shard initially thought this would take some time, but to his surprise, in less than five minutes, they both saw the door standing abruptly.
