Here's why:
* Participles can end in -ing (present participle) or -ed (past participle). Examples:
* Walking (present participle)
* Walked (past participle)
* Gerunds always end in -ing. They are verb forms used as nouns. Examples:
* Walking is good exercise.
* I enjoy reading.
The sentence "All -ing words are participles" is incorrect. While all participles ending in -ing are also gerunds, not all -ing words are participles. For instance, "running" can be a gerund, but it can also be a present participle (e.g., "The running water was refreshing").