Imagine a factory has created two identical blocks of wood. One of the blocks is stationary next to you, while the other has been set in motion and is speeding by you. What would you say about the density of the moving block compared to the stationary block?
Hint: The density of an object is its mass divided by its volume.
The block is Lorenz contracted (in the direction of the speed, 2 other measurements stay the same), so the volume decreases by a factor of gamma. The mass is increased by a factor of gamma. Which means that the density, whici is mass divided by volume, increases by a factor of gamma^2.