Play Chapter 6 "The Fall" and reach the 1958 "28" room. There is a cube is trapped in a glass box and Blue Gel streaming out of a pipe in one corner in this room. Save the game. Shoot a portal under the Gel and over the crate to coat the cube. The cube will bounce until it breaks the glass. It will eventually break out of the glass. Catch this cube before it touches the ground to earn the achievement. Note: Try standing between the entrance and the gel spout.