Chapter 3, Mishna 1
We may spend maaser sheni, second tithe funds, on fish and we may make an eruv techumin (for shifting our Shabbos dwelling to enable us to walk farther in one direction than otherwise permitted) with fish as well. May we make an eruv with cheap, insignificant produce like cress or sweet clover? Rav said yes, but we may not make an eruv with green grain or unripe dates. The Gemara objects: but sweet clover is unsuitable for some people because it impairs the reproductive system! Nevertheless it may be used because it is suitable for others. And is it true that we may not use green grain for an eruv? Rav himself said we may! What changed? And what blessing do we say when we eat green grain? Beautiful story about Rabbi Zeira honoring Sages when he was too tired to learn anymore himself. Perhaps it was because he was so tired that he debated a young school child on the correct blessings for green grain and the parasitic plant, dodder… and lost!