Sometimes when time is short, we use the tried and true counting-off method to make groups. At other times, we use more-elaborate activities, such as the Colored Dot Game described earlier. Frequently, we use categories to create pairings or small groups. Locate two other students who have the same number of siblings, the same color socks you are wearing, birthdays in the same season, or the same number of letters as your first name. The possibilities are endless.
Students enjoy suggesting categories too.
When recess is peaceful and active, children come back to the classroom refreshed and able to focus. When recess hasn't been a time of activity and fun or is full of unresolved conflicts, children come back to the classroom simmering. The spillover keeps them from hearing the math lesson or the read-aloud. That's a big part of the why to teach recess. This varies a lot, depending on arrangements for recess in different schools.