If you’ve ever slid across your lawn to catch a ball, you know that grass can be an unforgiving surface. Despite its innocuous appearance, grass blades can be quite abrasive on skin, causing an unpleasant burning or itching sensation. This can be true even if you’re reclining on it, trying to soak in nature even as nature apparently wants you to relocate somewhere else.

Why does grass treat us like this? How can something that looks so inviting and harmless create such searing discomfort?

There are a few reasons. While grass may look and feel soft, it can actually cause micro-abrasions on your skin. The edges of the grass itself may damage skin, but there’s another factor on their surface. While not visible to the eye, blades of grass are covered in tiny hairs known as trichomes that can irritate soft tissue. The trichomes help make grass and other plants unappetizing to animals. (Because when you can’t move, having other means of defense helps.) While not generally harmful, the tiny cuts allow sweat to seep in to the tissue, making for literal salt on the wound that can provoke a stinging sensation.

While mostly harmless, trichomes can sometimes be more formidable. The stinging nettle, for example, has trichomes that act like tiny hypodermic needles, which can inject histamine and other irritants to discourage foragers.

Grass also has a good supply of pollen covering it, both its own and from neighboring growth. For people with pollen allergies, this can produce itching and welts that prove especially irritating once the skin has been damaged by the trichomes.

And sometimes, it’s not the grass itself but what it’s harboring. Mites, fleas, and other bugs can be present on grass, prompting an allergic reaction when they bite.

Fortunately, there are some steps you can take to avoid itching. If you know you’re going to be sitting in grass for a picnic, opt for wearing pants instead of shorts. If pollen bothers you, check pollen counts before spending a significant amount of time outdoors and wash your clothes after coming in. Just don't expect it to be too hospitable to your bare skin. Remember—they’re called blades of grass for a reason.