Lungs aren't made for breathing under water and other such stuff

Last night when my 5 year old was taking her bath I heard this sputtering and gasping noise like she was drowning.  I ran into the bathroom; keep in mind that she broke her arm while riding her bike about 4 weeks ago so her arm is in a cast all the way past her elbow and she just got a waterproof cast on Monday so she has been enjoying being able to put her arm in the water.  But I thought she had lost her balance and was going to drown so I ran in there.   By the time I got to the bathroom she was just sitting there with an exhilarated look on her face.  I asked her, "what were you doing?" and she replied, "I wanted to see if I could breathe under water."