My 3 year old daughter's eyes have been goopy in the morning when she wakes up?

This morning they were almost completely stuck shut. I had to wipe them with a warm washcloth, which is not easy if you know 3 year olds. She's has a cold, but hasnt been rubbinb her eyes so they don't seem to bothering her. I remember my eyes doing the same thing. What could it be?