Here's why:

* Frame of Reference: A frame of reference is a coordinate system used to describe the position and motion of an object. It's like a viewpoint from which you observe something.

* Relative Motion: The motion of an object is always relative to another object or frame of reference. For example:

* A person walking on a train appears to be moving to someone standing on the ground.

* The same person appears stationary to someone else sitting on the train.

* Galilean Relativity: This principle states that the laws of physics are the same for all observers in uniform motion. However, the description of motion itself can vary depending on the observer's frame of reference.

Examples:

* A ball thrown straight up: To someone on the ground, the ball goes straight up and then straight down. To someone on a moving train, the ball appears to follow a curved path.

* A car moving at a constant speed: To a passenger in the car, the car seems stationary. To someone standing on the road, the car is moving.

Key Takeaway: How we perceive motion depends on our own position and movement relative to the object we're observing.