Why does the grass get so cold?

With no sun shining, the grass gives its warmth away up into the sky. On a clear night, there are no clouds to send that warmth back down, so the grass cools fast. That's why the most dew comes after clear, calm nights.

Why is the grass wet in the morning?

Roo answers for ages 4–9, then goes 1 step deeper each time a child asks why.

Roo says

It didn't rain, but the grass is sparkly wet. The air always has some water in it, as a gas too tiny to see. Overnight, the grass gets cold. When the damp air touches the cold grass, the water turns into tiny drops. Those drops are called dew.

For ages 4–7

Checked against What is dew? (Met Office)
Why does the grass get so cold?

With no sun shining, the grass gives its warmth away up into the sky. On a clear night, there are no clouds to send that warmth back down, so the grass cools fast. That's why the most dew comes after clear, calm nights.

For ages 6–9

Checked against Dew and frost (National Weather Service)