If you measure a wheel with a ruler, and the wheel is moving relative to the ruler, then your measurement assumes that both ends of a piece of the wheel line up with both ends of a piece of the ruler at the same time. Whether these events happen at the same time, and therefore whether this is a measurement of the wheel, are different depending on the frame of reference.
If you measure a wheel with a ruler, and the wheel is moving relative to the ruler, then your measurement assumes that both ends of a piece of the wheel line up with both ends of a piece of the ruler at the same time. Whether these events happen at the same time, and therefore whether this is a measurement of the wheel, are different depending on the frame of reference.