Shabbos, Chapter 9, Mishnah 2. This mishnah continues the side track running through the chapter, which brings sources from Scripture - various verses - for various halachos.
"Minayin lisfinah shehi tehorah" - How do we know that a ship is tahor:
The mishnah asks: "Minayin lisfinah shehi tehorah?" - how do we know that a ship does not become tamei at all? "Shene'emar: 'Derech oniyah b'lev yam'" - the way of a ship is in the heart of the sea, and from here we derive that a ship is treated like the sea itself: just as the sea does not become tamei, so too a ship does not become tamei.
The Gemara notes that seemingly this verse is not needed. After all, a ship is usually very large, and the halachah regarding wooden vessels - which are compared to a sack, whose purpose is to carry its contents - is that a vessel so large that it cannot be carried when it is full does not become tamei in the first place.
Which ship, then, is the mishnah speaking about? There are two possibilities:
An earthenware ship: earthenware vessels are not included in the comparison to a sack, and that halachah does not apply to them. An earthenware ship would therefore have become tamei, were it not for the verse "derech oniyah b'lev yam" teaching us otherwise.
A Jordan boat: a very small boat used on the Jordan River, which is a small river. Since it is small, it would have become tamei even though it is made of wood, were it not for that very same verse.
The Laws of Kilayim - the Garden Bed:
The mishnah continues with the laws of kilayim, according to which one may not sow different kinds of vegetables together, and if they are separated in a certain way it is permitted. It asks: "Minayin la'arugah shehi shishah al shishah tefachim, shezor'in b'sochah chamishah zer'onin - arba'ah b'arba ruchos ha'arugah v'echad ba'emtza?" That is, in a garden bed measuring six tefachim by six tefachim one may sow five different kinds: four at the four sides of the bed, and one in the middle.
The Source in the Verse:
"Shene'emar: 'Ki ka'aretz totzi tzimchah uch'ganah zeru'eha satzmiach'" - each of the words in the verse refers to another kind:
"Totzi" - the first kind.
"Tzimchah" - the second kind.
"Zeru'eha" - two more, a third and a fourth. In the words of the mishnah: "zera lo ne'emar ela zeru'eha" - the verse does not use the singular but the plural, and from here we learn that two seeds are meant.
"Satzmiach" - the fifth kind.
From here we have the hint that one may sow five different kinds of seeds in a single bed of six tefachim by six tefachim.
Ordinarily a distance of at least a tefach and a half is required between one vegetable and another. Our mishnah is dealing with a case in which, even had the kinds been a tefach and a half apart from one another, we would not have permitted more than five. For once there are more than five kinds in the bed, it looks like a large mixture and has the appearance of kilayim, and it is therefore forbidden because of maris ayin - even though kilayim on a Torah level applies only when the seeds are actually sown together.