Have you heard of a froghopper? They are small brown insects that can jump a long way and feed on plant sap. They are also called spittlebugs because of the spittle that they produce to protect their baby. The young froghopper nymph lives inside a mass of foam, often called 'cuckoo spit.'
Take a careful look on plants and grass when you are on your next walk, you might find some froghopper nymphs in their cuckoo spit.