Hi, I am trying to solve this using only method 2 to iterate. I've been able to iterate by list and then iterate by index, but how do I iterate using indices only?

```
def flatten(lists):
results = []
for items in lists:
for i in range(len(items)):
print items[i]
results.append(items[i])
return results
print flatten(n)
```

What I am trying to do:

```
def flatten(lists):
results = []
for i in range(len(lists)):
for i in range(len(lists[i])):
print lists[i]
results.append(lists[i])
return results
print flatten(n)
```

Is this possible to do? Would someone mind explaining 'IndexError: list index out of range'? Thanks; very much appreciated.