This is a patch of Common chickweed (Stellaria media). Next is a patch of Mouse-ear chickweed (Cerastium fontanum).
Similarities abound with these two plants. And there are some distinct differences. Let’s start with how they are alike. Both have leaves arranged oppositely along the stem.
When you pull gently on the stems of each plant, they stretch apart to reveal an inner core.
Both chickweeds grow in the same habitat. Both have the same low-to-the-ground and spreading overall shape.
Now for some differences. The leaves are differently-shaped.
The older leaves of Common chickweed have stems (peduncles) while all the Mouse-ear chickweed leaves are sessile (no stems). The stems of Common chickweed have a line of hairs between each node while the stems of Mouse-ear chickweed are hairy all over.
The flowers have two distinguishing differences. The five petals of Common chickweed are deeply divided — so much so that it looks like ten petals. The five petals of Mouse-ear chickweed are divided — but not so deeply — usually no more than half the length of the overall petal.
The second difference is the number of styles within each flower. For this, you will have to get very close to the flower. A hand lens might even be necessary.
Chickweeds in the Stellaria genus have three styles — as in Common chickweed (Stellaria media). Chickweeds in the Cerastium genus have five styles — as in the Mouse-ear chickweed (Cerastium fontanum).