The realization that tennis balls are livelier on warm/hot days. Actually, I first noticed that balls felt hard (esp Dunlop balls) and did not bounce very well on cold days/nights. The air pressure in a pressurized ball is highly dependent on temperature. As the ball warms up on a cold night, the bounce improves somewhat. This ball liveliness can affect your judgement of the bounce if you are not aware of it.
If u have one foot stuck in yesterday & one foot stuck in tomorrow, u are peeing on today.