I'm British and only learnt this after studying Japanese which encodes vowel length in its writing system, eg, uncle: おじさん, grandfather: おじいさん. I refused to believe that English had vowel length, and was shocked to learn that my ears are actually very capable of hearing it. The other classic example in English is "merry" versus "Mary", but I couldn't find a whole sentence that is distinguished merely by a vowel length, so I made one (at least for my non-rhotic British accent):
Look how many clothes he shared!
Look how many clothes he shed!