In group out group

On Wensday our class did this project where the teacher divided the class into two groups.One group was people who was wearing black shirts. The other group was people who didn't wear black shirts. More than half the class didn't wear black shirts. When we divided up we had to write down reasons that they did or didn't wear black shirts. Some people said that the reason that they wore black shirts was because either that is the only color they had or it was the first thing they saw and picked out. No one knows exactly why people wore black shirts.