We have left a few unknown Chinese characters before we can fully understand the first and most important sentence of the Confucianism classic： <<论语>> - 学而时习之，不亦说乎？With a full understanding of the original meaning of each of these characters, we will have a much better comprehension of this sentence.
As the title, the character today is 爻. Its ancient scripts are pretty much the same as what we write today: (image taken from: http://vividict.com/)
|Evolution history of 爻|
It is pretty hard to know what it is if you do not have in-depth knowledge of Chinese culture. 爻 is the most important tools for ancient Chinese wizard to calculate or predict what to happen in the future:
爻 is composed of two 乂, which is the drawing of two sticks crossing each other. Two sticks represents Ying (阴) and Yang (阳) .
So 爻 is the most basic element of the universe: a pair of Ying (阴) and Yang (阳). And it is also used to represent the most basic element of 八卦 / Bagua (https://en.wikipedia.org/wiki/Bagua) or 易经 / I Ching (https://en.wikipedia.org/wiki/I_Ching).
That is all for 爻, now it is time to enjoy a Chinese calligraphy with it inside: