Everyone will know this one.
Common Daisy - Eòinean - Bellis Perennis
Depending on your viewpoint these will either be the flower of happy childhood memories or an invasive weed ruining your carefullly cultivated green lawn. I'm 100% in the former camp.
It has been postulated that the word "daisy" comes from "day's eye" because the flower (in common with many others) closes overnight, before opening again in the morning.