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.
As seen from my stationary perspective, the density of the moving block should be higher, since in the equation: density = mass/volume, the mass will be multiplied by gamma and therefore be larger, while the volume will be smaller, because one of the block’s dimensions (the one in the direction of motion) will be contracted (divided by gamma).