Start out with both containers about 3/4 full of water. Show the group the orange and explain to them that this represents a youth like them and you can name it "Joe" or "Susie" etc. - - tell them that it represents "Joe" or "Susie" when they do all of the daily things that they have been told will keep them safe, or "afloat" in life such as, daily prayer, scripture study, getting along well with family members, church attendance, etc. etc. etc...
Place the orange in the first container - - it will float at the top of the water.
Then pick up the second orange and call it by name and say something to the effect of "Susie" didn't say her prayers this morning but she was really in a hurry (and peel off a chunk of the peel);
"Susie" didn't attend YW this week because she had a test to study for (peel off another piece);
"Susie" had a huge fight with her mom about going to the high school dance with a date - - she knows she's wrong because she's not 16 yet, but her mom could have let her go anyway (peel off another chunk);
"Susie" told her geometry teacher that she'd lost her homework and would redo it but knew she'd never finished it the first time (peel away another chunk)
When the orange is about half peeled, ask the group how they think "Susie" is doing - is she staying afloat by staying true to the guidance and directors that the Lord has given her? Place the orange in the second container of water and it will sink but not all the way.
Take it out of the water and continue with "Susie" was too tired to get up for early morning seminary and figured that missing one day surely wouldn't hurt (peel off a piece) - -
-Continue on with these seemingly small infractions until you have peeled the entire orange - - place it in the water and it will sink.
Make the point that an orange peel has very tiny air pockets in it. These small, barely noticeable air pockets are what keeps the orange afloat. They have small, routine directors in their lives (such as daily prayer, seminary, church and youth night attendance, scripture study, reading the new era, obeying the guidance given by their parents, leaders and Strength of Youth pamphlet, etc.) that keep them afloat but if they "peel them off" by not doing them or disregarding themselves, they are sinking themselves slowly but surely.
End on a positive note by reminding them to keep themselves afloat by doing the small things, like they're doing right now. Each attended meeting (like this one) can be like an airpocket that can help keep them afloat.