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When a thumbtack cannot be pulled off from a message board,
an elastic iron sheet can be used to press the center of a thumbtack, and its edge will bounce.
One can easily pull a thumbtack off by the arc of its edge.
Quite a few people get injured when they step on fallen thumbtacks due to negligence.
The arc shape of nicks on the edge of a thumbtack can reduce the area of a thumbtack.
When it falls, it touches the ground with its three points, and its sharp pin will not face upward to hurt anyone
YUTI HUANG SHUOTING CHUANG