I am in the sky but also in the ground. When you study me, no matter how long, I will always end with an f. I may be in your yard but not in your house. What am I?

Why this works

This riddle cleverly plays with the different contexts in which the word "leaf" appears. First, "in the sky" refers to leaves on trees that reach upwards, while "in the ground" alludes to fallen leaves on the forest floor or ground. The phrase "always end with an f" hints at the spelling of the word "leaf," which indeed ends with the letter 'f.' Lastly, leaves can be found in your yard as part of outdoor plants but typically aren't found inside your house, completing the playful contrast in the riddle.