His mind was suddenly taken off that problem by a small scratching sound to his right. He stopped breathing again as he looked in that direction.

Just along the branch, a small gray robot was trying to stand, but every time it pulled itself to its feet, it went toppling woozily onto its back again. It was almost comical, but Alex didn’t feel like laughing. The scratching sound was the noise of its hook-hand, digging viciously into the bark to stop it falling.

Flier. The word buzzed into Alex’s mind with a sting. The robot was badly mangled. One of its razor-edged wings had been bent, almost torn off. On its head, a tiny propeller jerked uselessly, then died. Its lamp-eyes gave only the faintest amber glow. But the hook and scalpel blade that served as hands still seemed sharp enough.

The thing had given up trying to stand. It sat staring dully at him. Alex noticed a new detail: around its hook-arm hung a small patch of black cloth. Before he could think further about it, his attention was snagged by a more pressing point. The flier had started crawling toward him.

It clawed at the branch, sinking its hook into the wood with a hollow little thunk and pulled itself forward. Then repeated the maneuver, drawing closer in tiny increments.

Thunk.

Alex tried a kick at it, swinging one leg up. His foot hit the underside of the branch, and he swayed terribly backward as it dropped away again like an anchor. He held tighter. With the dead weight around his feet, it was all he could do to hang there and watch the robot closing in.

Thunk.

Thunk. Nearer.

Thunk. Seven inches away.

It raised its scalpel-arm as though testing it, making a slash at the air.

Thunk. Five inches.

Slash-slash.

THUNK.

This was a sound of a different, more violent order. With it came an explosion of splinters that Alex had to duck to avoid. When he turned back, the little machine was lifeless, lying impaled by one of the three sharp prongs of the grappling hook that had embedded itself into the branch about three inches from Alex’s face.

“Sorry, old chap,” his grandfather’s voice sailed up from below. “That was a little closer to you than intended.”

The rope tied to the hook tightened as Alex’s grandfather started hauling himself upward. The branch trembled and groaned.

“And that’s why,” the old man grunted as he swung himself over the limb, “it always pays to ask permission.” He sat straddling the branch with his back against the trunk, catching his breath. “If you had asked me, I could have told you that the spring-heels are rather carefully calibrated for a specific user’s weight and height. In this case: mine. Plus, I think Albert might have been a wee bit overenthusiastic about tightening them up.”