May have been mentioned somewhere else, but when right clicking on a block with a nonplaceable item or empty hand a stack of whatver item is in my retical is moved to my hotbar and my hand. The item must already be in my inventory. (eg. While sand is in my inventory, I'm using a bucket to move water blocks in a shallow pool, my retical is on blocks of sand, and after a few right clicks I find the stack of sand in ym inventory is now in my hotbar and in my hand being place.