Name two flightless birds.

Explanation

### Step-by-Step Solution:

1. **Understanding Flightless Birds**:
- Flightless birds are species that have lost the ability to fly. This can be due to various evolutionary adaptations.

2. **Characteristics of Flightless Birds**:
- Flightless birds typically have solid and heavy bones, unlike flying birds which have hollow and lightweight bones.
- They also lack a keel, which is an extension of the breastbone that serves as an attachment point for flight muscles.
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