It's hard to think any song from The Flintstones could possibly be catchier than the theme song. But then in the animated series' final season, the very first episode "No Biz Like Show Biz" delivered a sugar-sweet new contender. 

"No Biz Like Show Biz" builds up to a concert Fred dreams up where Pebbles and Bamm-Bamm are rockstars performing a song called "Open Up Your Heart (And Let the Sunshine In)." With Bamm-Bamm on the drums and Pebbles on a two-string guitar, the 1965 episode dusted off an old song from 1954, and it became such a sensation that through the rest of the season, this clip was often used at the closing credits of the cartoon, replacing the theme song.

The original version of "Open Up Your Heart (And Let the Sunshine In)" hit No. 8 on the Billboard charts in 1954, performed by its writer Stuart Hamblen and his band the Cowboy Church Sunday School. Fans of the Rosemary Clooney hit "This Ole House" may recognize the name Stuart Hamblen from their record labels, because he wrote that tune, too.

But "Open Up Your Heart (And Let the Sunshine In)" reached fans of all ages through The Flintstones, which meant in addition to covers done by country legends like Kitty Wells, the song was also belted out by kids who liked singing its refrain, "Smilers never lose and frowners never win / So let the sunshine in, face it with a grin."

But the song's legacy didn't end right there. As fans of The Flintstones know, Pebbles and Bamm-Bamm as rockers caused such a craze, they went on to form a band when they became teens. The Bedrock Rockers first appeared in the 1970s cartoon show The Pebbles & Bamm-Bamm Show, and then became a fixture of The Flintstone Comedy Hour. On those shows, they performed plenty of original pop songs, including "Summertime Girl," "Summertime Man" and "Yabba Dabba Doozie," but as popular as some of these became, "Open Up Your Heart (And Let the Sunshine In)" actually went on to inspire a much bigger hit four years after it was memorably performed on The Flintstones.

In 1969, the 5th Dimension released what Billboard still considers one of the "Greatest Songs of All Time," in "Aquarius/Let the Sunshine In." Any kid who tuned into The Flintstones in 1965 likely recognized the second half of the hit song title, using the same lyrics as Pebbles and Bamm-Bamm sang in the Hamblen hit "Open Up Your Heart (And Let the Sunshine In)."

The reference becomes undeniable by the time you get to the end of the 5th Dimension hit, when the chorus fades out singing, "Let the sunshine (And let the sunshine on in) / Let the sunshine in (Open up your heart) / The sunshine in (Let it shine on in)."

So if you ever wondered how to connect the modern Stone Age family to the "Age of Aquarius," there it is.

Although the Cowboy Church Sunday School took "Open Up Your Heart (And Let the Sunshine In)" up the charts, the most well-known recording today remains the version we all heard on The Flintstones. It was performed by Rebecca Page, who voices Pebbles, and her mother Ricky Page, who in the song voices Bamm-Bamm's parts. This mother-daughter duo eventually went on to form their own bands called The Bermudas and The Majorettes, but again, The Flintstones song is the one that stuck.

Do you remember singing along with this song at the end of the later episodes of The Flintstones as a kid?