Donna entered the console room holding a record in a dark pink envelope. the Doctor was fully immersed in the repairs.
Donna asked: "Figured out why your box can't tell the difference between accessories and pestilence?"
"Hey, bats are actually extremely intelligent creatures. They hunt by sonar alone, like a submarine, and almost only eat fruit. They didn't mean to hurt you, your hair probably looked like two persimons to them. You had your hair up." the Doctor countered.
"Whatever, Spaceman. Wanna hear some music? I found an old record." Donna asked.
"Where would you get a record?.. Oh, the storage room." The Doctor wondered for a second how he felt about Donna having been in there. He decided it was fine. He didn't need to keep secrets from her. At least not a lot.
"Sure! I miss Gallifreyan music. It's really quite extraordinary, Donna!" the Doctor said with a smile.
Donna walked over to the record player, put the record in, and pressed play. No music was heard. At least, Donna didn't hear any.
The Doctor instantly realized what song that was. He hadn't heard it since he was a Time Tot. He thought it ironic that this song played at this specific time.
He'd been needing to pee a bit during the trip to the bat planet, but had chosen to ignore with all the focus he had, to avoid awkwardness while all these adorable bats flew around him. Until now, he had held it in for about 2-3 hours, including the time spent sort-of fixing the console.
His bladder and sphincter nerves reacted instantly to the soundwaves. The pressure increased and he could feel the muscles containing the liquid vibrating as if to warn him of impending release.
He knew he had to act NOW!
He urgently yelled at Donna: "STOP THE MUSIC!". Donna was confused, but did as he asked.
"Thanks Donna", he said, once he managed to compose himself, and re-tighten his muscles.
"What's the matter?", Donna asked.
Donna didn't believe it for one second: "Oh, don't give me that nonsense. You just yelled bloody murder at me to stop the record. Tell me. What's wrong with the music? Or lack of music?"
"I just hate that song. You see, Time Lords are telepathic, we can talk to each other through our minds. Our songs are played through telepathic soundwaves. Your human ears can't detect them. Just, don't play that again okay?"
"Okay!", Donna said, while moving her finger towards the "play button. She knew it was more than hatred involved, and wouldn't give up her search for answers that easily.
The Doctor saw Donna about to play it again, and panicked.
"DONNA STOP, please don't play it, please, just don't, please!" he yelled.
Donna retreated instantly. Now she had him.
"Okay, I won't. Just tell me this? Why are so scared of the song?", she asked.
"What? I'm not scared", he lied.
"Oh really. You practically begged me not to play it. Twice. You want me not to play it? Then tell me what's wrong with the song.", Donna said, with her finger hovering over the play button.
Now, the Doctor knew he had no choice. "Alright! I'll tell you.. Let's sit down."
They sat down next to the console, facing each other.
The Doctor took a deep breath, and started.
"It's a toilet training song."
"What?", Donna said.
"You know when kids are three year olds, they start toilet training?"
"And parents sometimes use songs to, help the process.", he explained.
"Yeah", Donna replied.
"Well, that song", he said, pointing to the record, "is what my mother played for me, when I was that age."
Donna started to understand. "Your mother used that song on you, when you were a toddler?"
"Well, on my planet we call them Time Tots, but yes."
"Hold on, I still don't get it. Why are you so scared of a toilet training song?", Donna asked.
At this, the Doctor blushed, and looked down with his eyes closed. This was it. The reveal of the most embarrassing secret he knew. Well, one of them.
"Ehm.. Because.. it still works.", The Doctor admitted.
Donna suddenly realized what the Doctor just said. She burst out laughing. The Doctor covered his face in his hands.
"Are you serious", Donna asked, containing her laughter as to not embarrass him unnecessarily.
"Oh yes!", he replied.
"That song still works on you?"
"Oh yes it does", The Doctor said, emphasizing how well..
"Now I see why you begged me to stop.. but how is that even possible?", she asked.
"I don't know, Donna! Doesn't matter. Can we drop it, please?", he begged, slightly humiliated.
"Yeah, of course. I'll put the record back where I found it.", she said, putting the record back in its case, and walking back towards the storage room.
Simultaneously the Doctor remembered what his body had wanted to do.
"Right, while you do that, I'm gonna pop to the loo! Be right back", he announced, running down an opposite corridor.
Donna smiled at that, and kept walking.
They both laughed at this funny event then rest of the day, with the Doctor only slightly annoyed.