Introduction[]
What's Inside? is an Achievement in Run 3.
Suggested Characters: 
Gameplay[]
The Lizard's jumping abilities are very useful here. Using its slow speed also helps in being able to precisely jump from box to box.
The Student's ability to hold herself in the air for a while before switching the gravity to bring herself to the top of the tunnel is also useful here. By jumping on one box and pressing the up key and holding it before switching gravity, you can use timing to know when to let go and thus being able to precisely jump from box to box.
The Child's ability to float down slowly grants the player ample time to plan which box to land on next, and how to jump off of it, as well as making it easier to land on the side of boxes. It is recommended that you hit the side of one of the boxes then float down, being able to float past the whole level.
This achievement can be made easier to beat by purchasing increased box spawn rates from the Shop.
If you get lucky, you can cheat this achievement with the Gentleman, by using the power cells to fly through the level.