Common yellow woodsorrel (Oxalis stricta) is a type of woodsorrel that is common in the United States and Europe; it also appears in the UK, but it is rare there. It is commonly found in meadows, lawns, and woodlands, and is usually recognized as being edible, at least by local children. It is also known by the names Yellow Woodsorrel, Common Yellow Oxalis, Upright Yellow Sorrel, Lemon Clover, Sourgrass, and Pickle Plant; my family simply calls it Oxila, although I suspect that we are just plain wrong.
Its leaves look like small clover leaves, with three heart-shaped leaves at the top of long stems. The flowers, on the other hand, look nothing like clover, being small yellow five-petaled flowers looking something like a flax or mallow blossom; however, for many of us the woodsorrel bloom is the prototypical field-flower, making comparisons counter-productive. You may also be familiar with plants of this description with white or pink flowers; these are closely related species of woodsorrel, and can also be eaten.
Because common yellow woodsorrel does not fix nitrogen in the soil, is not a particularly good plant for animal forage, tends to stand out a bit on a well-tended lawn, and has no qualms in invading garden beds, it is often seen as a weed; however, it is harmless and pretty, and not hard to weed out if you dislike it in your garden. It is generally a pleasant and pretty plant to find around. It tends to grow tall when young, later laying down and spreading along the ground as it ages, and then popping up again when it produces seeds. It grows well in full light or shade, folding up its leaves when the sun grows too hot (and again at night) and does well in sandy and nutrient-poor soil. It prefers alkaline, well-drained soils.
There are two things that make common yellow woodsorrel interesting enough that most of us will remember it from our childhood. First of all, it is edible, its stems being crunchy and sour, and somewhat lemony in flavor. Some people will add it to salads or drinks (for example, it has been used to flavor barley water); the blooms can also be eaten, making it a quite striking addition to a salad. However, it is most often eaten by children. It is worth noting that its flavor comes from oxalic acid, which is technically a poison; however, you would have to eat a tremendous amount in order to cause any health problems. You should probably not gorge yourself on any species of woodsorrel, and you should avoid it if you have impaired kidney function.
Unlike many species of woodsorrel, common yellow woodsorrel has distinctive seedpods that pop explosively to release the seeds. They tend to do this when disturbed, for example when you walk across the lawn. These seeds can fly for 10 feet or more, and tend to come in bursts as multiple seedpods go off at the same time. The seed pods look like small rockets (or, perhaps, tiny corncobs), and tend to be on stalks standing high, making them quite visible and tempting. If you are very very careful, you can take an unexploded seedpod off of the plants and carry it around; this is not particularly useful, as the explosions cannot be aimed and are very quiet, but it is amusing enough that it kept some of us occupied for hours as a child.