Licking lead once is completely fine, just don't make a habit of it

Come on, it can't be that bad if we build water pipes with it

;^)

Some of these would explode on contact with your tongue, yes.

Solid lead isn't really that dangerous. Like with a lot of other toxins, it's mostly only dangerous when ingested or inhaled. Licking a block or sheet of lead would cause a negligible amount of lead consumption.

While I'm not saying it was a great idea or anything, they used to make children's toys out of cast lead, and as long as little Timmy didn't bite his toy soldier's head off and swallow it they're perfectly safe, even if he was sucking on it sometimes. Hell, even if he did swallow it, he'd probably pass it with no issues.

Lead is dangerous when aerosolized, like with leaded gasoline, or in powder form, like with lead paint chips, because in these forms it is more easily absorbed by the body. Depending on the location, people can have lead bullets literally inside their bodies for decades without ever suffering from heavy metal poisoning (and when heavy metal poisoning does occur, it can take years to accumulate to dangerous levels).

Same goes for lots of other stuff - plaster, fiberglass, asbestos.