How does Piggy understand what the conch is and what it is capable of doing?

Piggy knows what a conch is right away,

"a conch, ever so expensive. I bet if you wanted to buy one. You'd have to pay pounds and pounds and pounds.....We can use this to call the others. Have a meeting.... How did your friend blow the conch?"

"He kind of spat," said Piggy. "My auntie wouldn't let me blow on account of my asthma. He said you blew from down here." Piggy laid a hand on his jutting abdomen. "You try, Ralph. You'll call the others."