There is a wood and a nail in the idiom. What is the answer?

From the picture, we can see that there is a nail in the wood and a red ruler outside, and the position is marked on the number "3", which easily reminds us of an idiom: alertness.

Interpretation: According to legend, Wang Xizhi wrote on the wooden board, and when the carpenter carved it, he found that three points of the handwriting penetrated into the wooden board (three points equals 1 cm). Describe that calligraphy is very powerful. Nowadays, the problem of metaphor analysis is very profound.

Source: Zhang Huaiguan's Book of Broken Wang Xizhi in the Tang Dynasty: "Wang Xizhi's book is a blessing version, which is cut by the workers." ; Poetry: Writing about the superiority of human beings and ghosts, stabbing greed and stabbing abuse into the wood —— Guo Moruo's comments on Strange Tales from a Lonely Studio.

The key to this problem is that on the red ruler, the depth of the nail can be seen in three red places, so there are wooden nails in the idiom, and the answer should be accurate.