What's the reasoning here?

So I heard a sound that under other circumstances would have made my heart very happy. The twins were laughing and giggling together. Oh, how sweet! The downside- the laughing and giggling was coming from the downstairs bathroom and was accompanied by the sound of splashing. I went to investigate and found the bathroom sink full of water and the twins sitting on the counter, feet in the sink, playing in the water. And one more thing. They were both completely naked. "We're playing in the swimming pool mommy!" they told me. I didn't try to figure out their reasoning, I just told them to head upstairs and continue their little "pool party" in the bathtub. Hopefully when we take them to a public swimming pool they will keep their swimsuits on.