On Chapter 6 "The Fall", reach the 1958 "28" room. There is a cube that's trapped in a glass box and Blue Gel streaming out of a pipe in one corner in this room. Save the game, then shoot a portal under the Gel and over the crate to coat the cube. The cube will bounce until it breaks the glass and eventually break out of the glass. Catch this cube before it touches the ground and the achievement is yours.. Note: Stand between the entrance and the gel spout.