Men’s Journal aims to feature only the best products and services.  If you buy something via one of our links, we may earn a commission. In This Story Matein Extra-Large Carry On Backpack, $24 (was $44) Check Price Why We Love This Deal I remember asking my 7th-grade geometry teacher the dreaded question: “When will we ever use this?” The rehearsed answer? “For you, John? Never.” But for those who go on to become creatives, designers , and engineers, these early geometry classes are critical. It's the reason a product like this seemingly simple backpack that Amazon sells for $24 has become such a massive hit. The Matein Extra-Large Carry On Backpack might sell for under $25, but it's become the site's No. 1 casual daypack for one reason: geometry. Much like my Kia Soul, it has a rather boxy build, yet, unlike my Kia Soul, the boxiness actually makes it more travel-friendly, with dimensions compatible with most airlines’ carry-on requirements.…