In Parshas Ki Seitzei we read (Deuteronomy 22:11), "לא תלבש שעטנז צמר ופשתים יחדו" - "You shall not wear shatneiz, wool and linen together."
This is the prohibition of shatneiz, wearing any garment containing a mixture of wool and linen (flax). The prohibition includes any use of such a material to warm the body (such as a blanket).
Shatneiz is one of the classic examples of a chok - a non-rational commandment. Nevertheless, as is the case with all of the chukim, there are always insights and lessons that can be drawn from this commandment.
The midrash (תנחומה בראשית ט', פרקי דר' אליעזר כ"א) connects the prohibition of shatneiz to the incident of Cain and Abel in Genesis 4. As the Torah tells us there, Cain brought an offering before God from the “fruit of the ground” and his brother, Abel, brought an offering from the choicest of his flock of sheep. God showed favor only to Abel’s offering because, as the commentaries explain, Abel brought from the best of his sheep, whereas Cain brought inferior material from his crops. Cain was angered by this and murdered his brother. The midrash states that the crop that Kayin brought as an offering was פשתים – flax. The midrash continues that this is the reason why it is forbidden to combine linen (which is made from flax) with wool (which comes from sheep) in our garments.
The Yismach Yisrael (Rav Yerachmiel Yisrael Yitzchak Danziger of Alexander, d.1910) uses this medrash to explain the deeper meaning of the verse from Eishes Chayil, the famous paean of praise for the Jewish woman written by King Solomon in Proverbs 31. The verse (Proverbs 31:13) states, "דרשה צמר ופשתים ותעש בחפץ כפיה" - "She seeks out wool and linen and her hands work willingly."
The Yismach Yisrael explains the words דרשה צמר ופשתים to mean, “She delves (דרש) into the meaning of wool and linen” - she seeks out the meaning of the prohibition of shatneiz taught by the medrash and she learns that the reason the offering of Abel found favor before God was that Abel made his offering with genuine happiness and enthusiasm, whereas Cain only gave from the poorer quality plants. Having learned this lesson and taking it to heart, ותעש בחפץ כפיה – “her hands work willingly;” she fulfills the commandments of God with happiness and enthusiasm.