机弦，也叫投石索、投石带、投石器、撂抛子，是一种投石工具，可以作为武器。由一个兜子和两端连接的绳子组成。使用时，把石块或土块放在兜子里面，一根绳子末端固定在手上上，另一根绳子末端拇指和食指捏住，然后甩动兜子，在适当的时刻松开拇指和食指，石子或者土块就会沿切线飞出，这样可以比人用手扔石头远得多。经过一定的训练，可以将石子或者土块准确地击向目标。

历史上机弦被用于打猎、战斗、放牧等。直到现在，中国西北地区牧羊人还在使用它来驱赶羊群。

大卫就是用机弦投石击倒巨人歌利亚的。《圣经·撒母耳记上17章》里记载“大卫用机弦将石子击中歌利亚的额，歌利亚就仆倒，面伏于地。大卫将歌利亚的刀从鞘中拔出来，用刀割了他的头，将他杀死。”

A sling is a projectile weapon typically used to throw a blunt projectile such as a stone, clay, or lead "sling-bullet". It is also known as the shepherd's sling.

A sling has a small cradle or pouch in the middle of two lengths of cord. The sling stone is placed in the pouch. The middle finger or thumb is placed through a loop on the end of one cord, and a tab at the end of the other cord is placed between the thumb and forefinger. The sling is swung in an arc, and the tab released at a precise moment. This frees the projectile to fly to the target. The sling essentially works by extending the length of a human arm, thus allowing stones to be thrown much farther than they could be by hand.