These seed-clusters look like they were tied on; Mother Nature is amazing at times! The Sycamore tree is one of my very favorite trees.
My wife dislikes them, calls them 'messy'. These seed pods are the result of the female flower being pollinated by the male flower.
Unlike the female flowers, the male flowers drop after the pollination completes; the female flower is what turns into these pods which stay on over winter and 'split' up in Spring.