Here's why:
* Participles are verb forms that act as adjectives, describing nouns. They usually end in "-ing" (present participle) or "-ed" (past participle). For example, "running water" or "broken chair."
* Gerunds are verb forms that function as nouns. They also end in "-ing" but act as subjects, objects, or objects of prepositions. For example, "Giggling is contagious" or "I enjoy giggling."
In the sentence "The children were giggling," "giggling" acts as a verb describing the children's action. It's not modifying a noun.