The little bubbles you can see on the surface of the eraser are compressed pieces of melamine, giving you the extra cut through power when breaking through and removing built-up grime.
The sponge itself is compressed with double the amount of melamine than our double size magic erasers when you squeeze it, it barely moves, that's how you know it's a double strength magic eraser, this type of eraser is built for tough cleaning, it's still a magic eraser sponge and it will break down, but it will last double the amount of time as an average good quality magic eraser.