The song is called, "waterfall.." By undertale, apparently Sans has a big surprise for frisk! Is he gunna tell her the truth or is it just a surprise for another friend zone that they have been having as always have a great day! Enjoy!
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*play song*
FRISK P.O.V
It felt a good day, I woke up but...sans wasn't around. I began to bring myself up and tried to look for sans, I looked everywhere for him but...no sign. I began to wake up Flowey which he didn't want to wake up so I gently picked him up and took him outside in the cold so he can realize he's been cold, "AHH cold!!! Huh what? Frisk! What was that for??" I began to take him back inside then sighed, "I'm looking for sans...have you seen him?" Flowey then gave me a smirk then said, "oh~ the skeleton...I don't know...tell me that you like him and maybe I could tell you~" I began to blush on how embarrassing this was.

I decided to play it cool but lie a little...even tho I hate to lie but mean it's embarrassing telling someone your secret and them telling others ya know, "I don't like him...like that, I like him as a friend a good friend heh...heh" Flowey gave me the eye then said, "well...then I can't tell you where he is..." I decided to try again to ask but Flowey kept denying, "unless if you don't admit it...then I won't tell you where he is...maybe a monster caught him or maybe his brother found him...I don't know" I began to get worried. It's true what is his brother got him or something...I began to sweat more and more.

I couldn't hold myself in so u just had to tell the truth..."okay...you got me...I love sans.." Flowey jumped up in happiness, "oh! I didn't know you love him! I thought you like like him" I began to blush so hard Flowey began to chuckle. "Okay...I'll tell you where he is but I gotta lead ya the way" I have him a nod and let Flowey on me to lead. Where we went it felt like circles till...I saw him a grilby's, "Flowey! Why didn't you say he was at grilby's!!!" Flowey started giggling, "sorry sans I didn't know you were here...I was a bit worried where ya went since...I knew you went to grilby's but not at this time" sans began chuckling in a cute way then smiled.

I took a seat from where is was standing then ate and talk, after a while sans wanted me to see something which made me confused, "be careful sweetheart!" I began to get a bit tired from all this walking and sans covering my eyes. I began to feel a bit confused till, "okay sweetheart you can open your eyes.." I open them and...what I saw was...so...so beautiful it was a waterfall and blue beautiful flowers mostly a lot of of this place was blue and dark and beautiful. I began glistening to the bells and the water noises, "sans...this is beautiful..." Sans began to chuckle and smiled, "I'm glad you like it sweetheart.." He said sweetly.

I began to look at the beauty of this till, "sweetheart?" I turned to see sans turning a bit red, "yes sans?"sans was going to say something till he had a better idea. He ran to the blue flower and whispered it then he told me to come here closer. I went closer to the flower and began to hear the beautiful whispers.....

SANS P.O.V
I woke up as early as I can to make this plan a success, I slowly got up and grabbed Flowey. Flowey began to panic a bit but calmed down, "I need your help...", "on what?...", I began to feel red and Flowey started to laugh, "sure...but you better promise to kiss her" that's when I began to hide myself and leave while Flowey was laughing. I went ahead and tried to prepare myself with a good spot to put frisk into tell her...I think and think...till! Oh the waterfall! I started to set up a good looking place where she enjoyed then left.

I went to grilby's and ate for a little then started to wait since I did told Flowey I was gunna go to grilby's. I chilled for a little till frisk came in, in a panic...I began to talk and hang out with her till it was time to show her. Frisk was so cute when she was very confused, when I open her eye she was lighted in excitement, "sans....it's beautiful!" She stared at the beauty of the water till I was going to ask her something...but I sorta ummm chicken out.

I went to the closes echo flower then whisper...I told frisk to come here and she was lighted on what I said, "frisk....something about you makes me wonder...you are the person I always want to be with...you made me change the moment I saw you....frisk...I love you" Frisk then looked at me then went to the next echo flower to whisper. When frisk was done she told me to listen closely, "i feel the same .." I got myself up and saw her smile. I pulled her closely then...closed my eyes to kiss her...even tho I don't have any.

It felt so good...her sweet soft lips, when we pulled away she felt happy. Me and frisk hugged within each other and talked,''so how come you have those flowers around you?" Frisk began to sigh and explain, "it's because...it means I'm dying..." I began to worry so much...will I ever get to see her? How long will she live? I need to protect her that's what! Frisk then look upon the farthest stars, "look! A shooting star!" I became confused on what she said...she explain with her sweet soft voice."its just...it's a star where you make beauty of wishes" I began to get what she mean till she went and whisper to the echo flower.

I tried to began to listen but it was too hard, when she was finished she told me to come closer, "I wish I could free everyone from this place.." I began to chuckle till she was confused, "that's my wish too!" We both began to giggle till we were tired. Frisk came and rested on me....I stroke her hair softy till she fell asleep...Flowey on the other hand kept smirking till he came up to me,"I ship it 10000% plus I get to see you guys married!" I began to get angry but calmed down after I sorta woke frisk up, but in the other hand...I'm glad I get to have a beautiful human like her.