Web
Legolas was intrigued by ducks. The traders had returned from Laketown the week before, bearing all sorts of useful and interesting things, a brace of ducks included. Legolas had never seen such creatures until that day. In particular, he had been fascinated by their feet.
'But why do they have this stuff here?' he asked Thranduil, pulling at the webbing between the toes. 'I do not have it, so why do they?'
'It helps them swim,' his father explained patiently. 'Both frogs and ducks have webbed feet, you see.'
Thus, it should have been no surprise to Thranduil, nor should it have confused him, when he came upon the bare-footed Legolas in the gardens one evening with a large wad of spider-web in his hand. His son did not look up as he approached, but continued with his task. He would pull a little off the ball of web, shove it between his toes and repeat this step until there was no more space between each toe.
'May I ask what you are doing?' Thranduil asked after observing this for some time.
Legolas looked up at him, then back down to his feet. 'I am a duck,' he announced, wriggling his toes. 'For I have webbed feet. I will go swimming now, and when I am hungry, I will duck my head underwater and pull up a fish to eat.'