Green lids in your lawn?

I need to find these lids in my round green grass they have something to do with water and the sprinkler Where would I be able to find them and someone can give me the real name for them

This could be heads of sprinkler covers when water is cut off. Or it may be to mark where you need a sprinkler head. You will need to walk slowly your yard and drag your shoe on the ground. When you find one or two, you'll know which direction to walk to find the others.

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