The earliest form of the character "笔" resembled a hand writing with a brush. Later, people added the character "竹" (bamboo) on the top of it, because the holder of the writing brush was made of bamboo. Thus it evolved into its modern form of "笔".
Jīntiān shàngkè wǒ jì le hěnduō bǐjì.
I took a lot of notes on class today.
Kěyǐ jiè wǒ yìzhī gāngbǐ ma?
Can I borrow your pen please?
Wǒ gāng mǎi le yígè bǐtǒng.
I just bought a pen container.