While there aren't many common sixth-grade science words that start with "y," here are a few possibilities, depending on the specific curriculum:

Commonly encountered:

* Yeast: A single-celled fungus used in baking and fermentation.

Less common, but possible:

* Yolk: The yellow, nutrient-rich part of an egg.

* Yttrium: A chemical element used in various technologies, though less common in sixth-grade science.

More advanced concepts:

* Young's modulus: A measure of a material's stiffness (likely beyond sixth grade, but good to know).

Remember, the specific vocabulary for sixth-grade science varies depending on the curriculum and textbooks.