Why is it that sometimes time flies by quickly and other times it drags agonizingly slow?

This phenomenon is called recalibration – it is the brain’s shift in time of information about the action closer to information about the result, due to which our conscious activity lies as if a little bit in the past.

Skin Rabbit

There is an interesting illusion called the skin rabbit. If you touch your wrist and after a short time your elbow, there is a sensation of touching the entire inside of your elbow, as if a rabbit had run through. The brain tends to combine sequential and spaced events into a definite trajectory.

Thus, the perception of time opens up a huge space for experimentation.