[quote name='VanillaGorilla']I have heard that if, during hot summer months, if you mow your lawn while it's really hot out, it can damage your lawn somehow. Not sure of the specifics, but maybe that is one of the reasons he was mowing it, before the sun comes out.[/quote]
It's true. You actually cause a lot of the moisture held in the leaves of grass to evaporate by mowing during the hottest part of the day. Best time to mow is in the morning or late afternoon/evening, or before rain.
Similarly, you should try to water outside plants at night or early in the morning, as it both conserves water and ensures the plants will have more chance to absorb it before it evaporates.