Time feels slippery in part because our perception of time changes. Time itself is a measure of change. It is not passing by us; we are passing by it, comparing the moment before and the moment after. We can perceive time either prospectively, while an experience is happening, or retrospectively, but time also varies depending on how we feel about an experience. Time flies when we are having fun in the moment, but our memories of such novel experiences can make them seem longer because there is more to look back on.
