अथ य इच्छेत् पुत्रो मे श्यामो लोहिताक्षो जायेत त्रीन् वेदाननुब्रुवीत सर्वमायुरियादिति उदौदनं पाचयित्वा सर्पिष्मन्तमश्नीयाताम् ।ईश्वरौ जनयितवै ॥६,४.१६ ॥
In plain words
Now he who wishes, "Let a son be born to me who is dark and red-eyed, who will recite three Vedas and attain the full term of life" — let the two of them have rice cooked in water prepared, and eat it with butter. They are then able to beget.
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
केवलमेव स्वाभाविकमोदनम् । उदग्ग्रहणम् अन्यप्रसङ्गनिवृत्त्यर्थम् ॥
In plain words
'Water-rice' simply means rice on its own, nothing added. Naming the water is only there to rule out milk, or curds, or anything else creeping in.
Close to the Sanskrit
'[Rice cooked in water]' means rice alone, in its own natural state. The mention of 'water' serves the purpose of excluding association with anything else.