Time
Most people find time a very abstract concept. This is mainly due to not realizing that they're able to control their perception of time relatively easily, albeit subconsciously. If you want to practice slowing down your perception of time, just do any kind of endurance exercise that doesn't require you to move. One excellent example is planks. If you haven't practiced planks before you'll be able to witness time slow to a crawl.
Of course the effects will weaken as your body strengthens as there won't be as much pressure.