坐井观天 (zuò jǐng guān tiān, 'a frog sitting at the bottom of a well looking at the sky') is a famous idiom about having a limited worldview. The full form is 井底之蛙 (jǐng dǐ zhī wā, 'the frog at the bottom of the well').
usage
井 appears in many compound words: 水井 (shuǐjǐng, water well), 油井 (yóujǐng, oil well), 矿井 (kuàngjǐng, mine shaft), 天井 (tiānjǐng, skywell/courtyard). The character 井 itself looks like the cross-section of a well.
grammar
The measure word for wells is 口 (kǒu, mouth), because a well has an opening like a mouth. This is the same 口 used for 口 of a pot or a 口 of people (in some dialects).
Xinhua indexes 井 under the 二 radical, picking the two crossing horizontals as the anchor. Pictographically 井 is the wooden cribbing around a well-mouth viewed from above; the two horizontals are the dictionary handle, with the verticals overlaid to complete the well-frame.
Two vertical strokes crossing the horizontals to form the four-sided wooden cribbing of a well-mouth seen from above. The crossed-stick pattern survives unchanged from oracle bone forms and is one of the clearest pictographs in the script.
