Frank frowned, thinking. Why had the aliens shut up? They’d replied exactly as the aliens had, showing that humans understood prime numbers, and—

No. All they’d shown is that humans can parrot things back at them. "Try continuing the sequence," said Frank.

Clete nodded, immediately seeing it as well. "They gave us the first five primes; give ’em the next five."

The captain nodded and lifted a small intercom handset off the wall, pulling it close to him. "Signaling room — continue the sequence. Give them the next five prime numbers."

"Sir, yes sir," said a staticky voice, "but, ah, sir, what are the next five?"

The captain looked at Frank, eyebrows lifted. Frank made a disgusted frown. Clete rolled his eyes. "Eleven, thirteen, seventeen, nineteen, and twenty-three," Frank said.

The captain repeated the numbers into the microphone. "Sir, yes sir," said the seaman’s voice.

"We better get up there," said Clete.

Frank nodded. "How do we get from here to where the controls for the searchlight are?"

"Come with me," said the captain. He led them to a circular metal stairwell and took them up to the radio room. As they entered, Frank saw the seaman who had been operating the light. He was a young white fellow, maybe nineteen, with a half centimeter of blond hair. "The aliens have started flashing again," he said.

"What was the sequence?" said Clete.

"They repeated back all ten prime numbers," the seaman said.

A wide grin spread across Frank’s face. "Contact."

The captain was looking out the window. "The Russian sub is signaling the ten numbers, too."

Frank pointed. "And here comes that damned cruise ship."

The yellow beacon started flashing again. One. Four. Nine. And then so many flashes that Frank lost track.

"It’s gotta be squares," said Clete. "One squared; two squared; three squared; four squared."