'dan sonra bilinen ikinci parçasıdır. Sözlerinin ve müziğinin kime ait olduğu bilinmemektedir. Klip adından da anlaşılabileceği gibi, mekan olarak kumsalda, kumdan yapılmış kalelerin arasında geçmektedir. Robin Sparkles ve sevgilisi (oynayan James Van Der Beek) klibin başrol oyuncularıdır. Aynı zamanda Let's go to the mall şarkısının klibinde gördüğümüz robot bu klipte de unutulmamıştır.
Parantez içindeki sözler klibin içinde geçen konuşmalar ya da vokallerdir. Didn't know how far I would fall My friends said I was a fool ("You're a fool" "Shut up") 'Cuz you were older and got kicked out of school ("You're so rad") Together we were gonna travel the globe From Alberta to Ontario But now I'm building Sandcastles in the sand Thought I could fly when you held my hand ("But now") Eternity turns to black and white It was the greatest week and a half of my life. We used to go to the beach Back when happiness was in our reach ("We were so close") On our favorite bench We'd sit and talk and you taught me to French You're good at pull-ups and have radical hair I hate my dad, he's so unfair ("You don't understand our love") But now I'm building Sandcastles in the sand Thought I could fly when I held your hand ("I was wrong") Eternity turns to black and white It was the greatest week and a half of my-- Life is crazy No one gets me Castles wash away Come back please I've changed my mind Let's go all the way ("I'm ready, let's do it") ("I miss you") Sandcastles in the sand Thought I could fly when I held your hand ("I thought you were the one") Sand castles wash away ("I'm ready, let's do it") and all that's left is some sand the next day Sand castles may be cute ("I'm on the pill now") But now all they do is remind me of you ("Let's go all the way") ("You said it'd be OK") ("I'm sorry")