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Test Tube and Fan never had much interaction until Let 'Er R.I.P.. This is when they discover that they have a lot in common, like being awkward. Ever since then, the two have become close friends, and are almost never seen without the other.
Episodes suggesting a friendship/relationship
In Let 'Er R.I.P., Test Tube tells Fan about the waning crescent, and Fan expresses how it's big, but looks small at an angle, just like him on the show. The two sit down as Fan confesses how awkward he is, and Test Tube relates to him. The two are attacked by MePhone5S and MePhone5C (which Test Tube initially expresses fascination for), but Fan tricks them into jumping off The Cliff, promptly causing their death as the duo look on, unfazed. While in the haunted house, the two start a discussion on the behavior of Lightbulb and Paintbrush until a desk and chair start to move. The two start to discuss the physics as Bow appears and calls them "nerds".
In Everything's A-OJ, Fan starts writing a blog post on Lightbulb's death, and asks Test Tube about how long it will take for her to shatter again. During the song, Test Tube is researching Fan's Egg as Fan writes on his computer. Later on when Paintbrush accidentally drops Fan's egg, Test Tube comes to it's rescue, to Fan's sweet relief, saying he is extremely grateful.
In Theft and Battery, both are ecstatic about going to MeCloud. Other than when Steve Cobs tries to take Fan's egg away and Test Tube is the first to step in. They don't interact much until their balloon is falling to the ground (courtesy of Toilet), and the two hold each other close when falling, once they're about to die, they hold hands.
In Mazed and Confused, Fan asks Test Tube to hold his egg for him while he does the maze with Lightbulb, making sure Lightbulb does not break it. Test Tube gladly accepts, saying staying away from people is her specialty, adding that Fan can count on her. Later, Test Tube is excitedly talking about how nice it was for Fan to trust Test Tube with the egg. Test Tube is seen near the end of the contest trying her best to protect the egg while she's burning. Fan exclaims if she's okay after the challenge. At the end of the episode where Spoiled Lemon is playing, Fan is seen holding a burned Test Tube while in utter surprise.
In Kick the Bucket (Part 2), Fan was trying to help Test Tube to throw the ball but since that she was thrown by Lightbulb into the lake, she did not understand what Fan means but successfully throw the ball and the Bright lights win.
Episodes suggesting a conflict
In Theft and Battery, Test Tube fails to cure MePhone using Fan's rice method, and she gets mad and yells at him, saying that he told her the strategy worked.
In Kick the Bucket (Part 1), whilst Fan is explaining to Paintbrush about how his theory was wrong and apologizing to him/her about that, Test Tube is constantly telling him to take the bucket whilst getting angrier and angrier that he is ignoring her.