English Explanation of Pirkei Avot 5:15

There are four types among those who sit before the sages: a sponge, a funnel, a strainer and a sieve.
A sponge, soaks up everything;
A funnel, takes in at one end and lets out at the other;
A strainer, which lets out the wine and retains the lees;
A sieve, which lets out the coarse meal and retains the choice flour.
Above in mishnah twelve we learned about four types of disciples. This mishnah is similar but instead of just describing the different types of students, it uses metaphors to compare them to different instruments in the kitchen. Furthermore, whereas that mishnah was considered with comprehension and retention, our mishnah is also concerned with the proper selection of what is worthy of retaining. The discussions in the mishnaic and talmudic study halls were probably very long, and no one could remember everything that was said. It would have been extremely important for the listeners to decide which statements were important and worthy of preservation and which were not.
A sponge, soaks up everything; The sponge soaks up all liquids, whether or not they are good or yucky. So too there are some students who remember everything that was said, whether or not it was reasonable and worthy of being remembered.
A funnel, takes in at one end and lets out at the other: The student who is like a funnel is able to take in all that is said, but he easily forgets it, like a funnel which lets everything out the other end.
A strainer, which lets out the wine and retains the lees: This is the most unfortunate type of student, who does not retain any of the well-reasoned valuable statements out, and only retains that which is not worthy of retention. It isn’t that he simply forgets the important things which were said, because he does remember some of what he has heard. Rather he cannot discern what was worthy of remembering in the first place.
A sieve, which lets out the coarse meal and retains the choice flour: After all of the bran is removed from the flour, the flour is passed through a sieve. What remains in the sieve are the larger, useful pieces of fine flour and the stuff that falls out is not useful [this is evidently not like the sieve we use today]. So too there are students who retain only that information which is important and remove the extraneous things that they have heard. This is obviously the best kind of student.