Cactuses

In honor of my New Years Resolution we went hiking.  As we began our hike my 4 yr old said "I don't want to get hurt by any cactuses."  I thought, this is a great teachable moment.  I explained that if there was more than 1 cactus then they are called cacti.  That some plural forms, which is more than 1, don't have a 's' at the end.  I did mention I was explaining this to my 4 yr old right?  Ok good.  As we got further along on our hike ds quips again, "There sure are a lot of cactis on this trail."  Oh well, he kind of got it.