How do you hot tub?

A few weeks back I was painting at the church with my friend Amy.  She had her kids Mason, age 4 and Avery, age 3 with her and I had Paige and Alaina.  Our kids were playing in the gym together.  Tommy had been watching them all morning, but had to attend a staff luncheon at noon. So because they played well together, we just left the kids in the gym and checked on them periodically.  They knew where we were and we told the older two to come get us if they needed anything. 

Well, in runs Mason saying, "They are taking their clothes off."  Amy said she would go and Mason stayed with me.  I honestly thought that Alaina needed her diaper changed and took her pants off.  Not all that rare around here.  The next thing I hear is Amy yelling for me to come quick.  I get to the gym and just laugh.  There stood Paige, Avery, and Alaina buck naked in the gym with several of the women on staff trying to help, but mostly laughing.  I thought I was going to pee my pants.  Glenda was trying to put underwear on Avery, but Avery is exclaiming that those aren't hers.  It was just hilarious.  After we got all the girls dressed and "talked to", we found out that Mason had suggested that they pretend the plastic wagon they were playing with was a hot tub and take their clothes off .  Poor guy kept saying, "I said pretend!"  He was mortified.  My question is. . . why does Mason think you take your clothes off to get in a hot tub?  We wear swimsuits!  ;)