What is the new skill that Tom and Joe learn from Huck?

Huck teaches Joe and Tom to smoke.

Tom said he wanted to learn to smoke, now. Joe caught at the idea and said he would like to try, too. So Huck made pipes and filled them. These novices had never smoked anything before but cigars made of grapevine, and they “bit” the tongue, and were not considered manly anyway.

Now they stretched themselves out on their elbows and began to puff, charily, and with slender confidence. The smoke had an unpleasant taste, and they gagged a little, but Tom said:

“Why, it’s just as easy! If I’d a knowed this was all, I’d a learnt long ago.”