Teseract
Imagine a universe of only two dimensions (like a sheet of paper) inhabited by flat beings. These beings know up, down, left, and right, but the concept of "upwards" (towards us in 3D) is completely incomprehensible to them.
If a three-dimensional object, like an apple, were to cross their paper world, the flat beings would not see the whole apple. They would only see cross-sections: a point that turns into a small circle, then a larger one, and finally disappears when the apple finishes crossing. Their 2D brains can only perceive "slices" of 3D reality.
If an object with four spatial dimensions were to enter our world, we would only see three-dimensional "slices" of it.





