What do you do for an encore after you’ve sold 20,000 flamethrowers? If you’re billionaire Elon Musk, you sell rocks.

But not just any rocks. These are rocks carved out by Musk’s tunneling venture, the Boring Company, and shaped for construction purposes. “Two people could build the outer walls of a small house in a day or so,” Musk said.

Musk launched his latest merchandising campaign tonight in a series of tweets — and as usual, answered questions from his avid fans:

New Boring Company merch coming soon. Lifesize LEGO-like interlocking bricks made from tunneling rock that you can use to create sculptures & buildings. Rated for California seismic loads, so super strong, but bored in the middle, like an aircraft wing spar, so not heavy. — Elon Musk (@elonmusk) March 26, 2018

First kit set will be ancient Egypt — pyramids, Sphinx, temple of Horus, etc — Elon Musk (@elonmusk) March 26, 2018

Guaranteed to be Flamethrower-proof! — Elon Musk (@elonmusk) March 26, 2018

Hey @elonmusk any way to turn this into affordable housing etc. in the areas of future drilling locations? Little to no transportation costs if contractors come to the drill sites to pick up material by the pound. https://t.co/Ni1QkLYUho — Leon (@LeonsArcade) March 26, 2018

Yeah, the boring bricks are interlocking with a precise surface finish, so two people could build the outer walls of a small house in a day or so — Elon Musk (@elonmusk) March 26, 2018

Wow. Is it biodegradable? What’s its impact factor on the environment? — Need More Wildlife (@DianaKFletcher) March 26, 2018

Uhh, it’s literally made of rock — Elon Musk (@elonmusk) March 26, 2018

Plan to ship worldwide? — Eniko Safrany-Hetei (@safranye) March 26, 2018

Yes — Elon Musk (@elonmusk) March 26, 2018

No word yet on pricing or availability — but if history is any guide, you can expect ordering information to show up on the Boring Company’s website.

The Boring Company has already referred to the brickmaking operation in its online FAQ, although the website makes it sound as if the bricks would be made from dirt rather than rock:

“In typical tunneling projects, excavated dirt is shipped offsite to disposal locations. This process is costly, time-consuming, noisy, and can be environmentally hazardous. The Boring Company is investigating technologies that will recycle the earth into useful bricks to be used to build structures. This is not a new concept, as buildings have been constructed from earth for thousands of years including, according to recent evidence, the Pyramids. These bricks can potentially be used as a portion of the tunnel lining itself, which is typically built from concrete. Since concrete production accounts for 4.5 percent of the world’s greenhouse gas emissions, earth bricks would reduce both environmental impact and tunneling costs.”

Whether it’s rocks or dirt, we assume that the raw material will be coming from the Boring Company’s tunneling operation in the Los Angeles.

Or maybe Musk is looking ahead to the stuff that’ll be coming up from tunnels in Maryland or Illinois. (At last report, Musk’s company is vying with one other finalist to build an express route from downtown Chicago to O’Hare International Airport.)

It’s not yet clear whether Musk’s bricks will be as economical as other building materials, either to make or to use in construction. But a couple of things are clear: First, Musk is definitely looking to merchandising as a means of support for the Boring Company. And there’s plenty of muck to merchandise.

No real surface is perfect, but it did pull about 250,000 lbs of muck rail cars out of a tunnel pic.twitter.com/wlKbLwd0f7 — Elon Musk (@elonmusk) March 22, 2018

Second, Musk has enough fans to guarantee that a good number of the bricks will be sold, even if they end up as glorified pet rocks rather than the stuff that pyramids are made of. January’s flamethrower sale already demonstrated that much.

And speaking of the Boring Company flamethrower (or should we say, “not a flamethrower”), Musk told a Twitter follower tonight that the first deliveries would probably be made in May.

There’s not yet any sign that Musk is joking, either about the flamethrowers or the rocks, but the acid test of Musk’s seriousness (or should we say, “not seriousness”) is likely to come on April Fool’s Day.