When it comes to tops: “like on like” and “different off different.”

Q: Lids from cans after opening the can — regular trash or recycling?

— D.D. in Middletown

A: When opening up a steel or tin can you have two options, D.D. You can either leave the lid somewhat attached and carefully bend it inside a little after rinsing the can out, or you can remove it entirely and, after rinsing, place it in the bottom of the can. A little pinch on the can may help it stay inside, too; though some cans are tougher to pinch shut than others. So this last step won’t always be practical.

Here’s the rule when it comes to tops, be they caps, lids, spray triggers, pumps, etc.: plastic back on plastic, metal back on metal, but metal off of glass (and then recycle both, separately). Another way to think about it is “like on like” and “different off different.” Note that with cartons, plastic caps can be fastened back onto their corresponding plastic spout or tossed in the trash — that one’s a bit tricky.

In some cases, your efforts to keep your metal lids inside your metal cans may be futile, D.D., but rest easy. If the lid comes out at some point in our sorting process it’s in the same boat as the loose metal lids you’ve taken off your glass jars. It’s not a big deal because these lids will eventually encounter a magnet that should pick most of them up. Keeping the lid attached or placing it in the bottom of the can is really just the safer thing to do, as it protects you from being cut by the lid’s potentially sharp edges, and it guarantees the lid winds up being sorted properly.

The extraction of tiny metal bottle caps from the recycling mix by the magnet is, admittedly, hit or miss, but if you can live with that, so can we. That’s important and worth repeating another way: If you just want to toss loose metal bottle caps in your recycling, go for it. If, however, you’re passionate about seeing 100 percent of your metal bottle caps make it through, here’s a trick: collect them in a steel/tin can, and when it’s half full, pinch the can shut and recycle it like that. Again, because some cans are tough to pinch shut, pick one that isn’t (or else you might need pliers).

Unlike metal tops, lone plastic tops have almost no chance of making it through the sorting system on their own. They’re usually too small or too flat, and they fall through various screens along the way. They really need a ride on their corresponding container. That means if you’ve been saving up your plastic caps separately (I sometimes get calls from residents who have been) these should just go in the trash and you should fasten plastic tops back onto their corresponding plastic containers going forward.

— Krystal Noiseux is the education and outreach manager at RIRRC. Send your question, including the city or town from where you're writing, to Trash Tutorial, Features Department, The Providence Journal, 75 Fountain St., Providence, RI 02902. Or email features@providencejournal.com and put "Trash Tutorial" in the subject field.