The Bath Song, apparently one of Bilbo's favorite bath-songs, is sung by Peregrin Took in Crickhollow while he, Sam Gamgee, and Frodo Baggins are bathing.
Three lines from the second verse are sung by Merry and Pippin in The Green Dragon Inn in the expanded version of The Fellowship of the Ring (film).

Sing hey! for the bath at close of day
that washes the weary mud away!
A loon is he that will not sing:
O! Water Hot is a noble thing!

O! Sweet is the sound of falling rain,
and the brook that leaps from hill to plain;
but better than rain or rippling streams
is Water Hot that smokes and steams.

O! Water cold we may pour at need
down a thirsty throat and be glad indeed;
but better is Beer if drink we lack,
and Water Hot poured down the back.

O! Water is fair that leaps on high
in a fountain white beneath the sky;
but never did fountain sound so sweet
as splashing Hot Water with my feet!

(Apparently Pippin added visual effects to the last verse, creating his own fountain of bath water and startling Frodo.)

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