How to show what 'sharing' means to two year olds
Two year olds will spontaneously share with the person they feel close to, e.g their mum. Which mother hasn't had a soggy biscuit almost pushed in their mouth, that is sharing, two year old style!
Sharing the way most grown-ups understand it means (for the adults!): Behave nicely by offering someone else what you have because that is how you stay on good terms with people. Well, two year olds DON"T GET THAT YET!
Toddlers don't think about staying on good terms with anybody. They are still busy exploring their world, everything is very exciting and therefore, any interruption is felt as an intrusion or attack and chances are they'll scream or hang on tight to what they have.
A much better approach is to use clear language, use the term taking turns. Taking turns can be demonstrated, e.g. in case of lose items one can demonstrate saying:
"One for you and one for me, one for you and one for me...."
In case of jumps on the trampolinne one may say: "Five jumps for Charlie and five jumps for John" then count the jumps, but don't be too precise with the jumps. Keep the counting presice, but if you count two jumps as "one" that is fine. More important than the actual jumps is the rhythm in which the words are said, for children respond to rhythm and chants.
Wait till a child shares spontaneously, then make the obvious statement: "Thank you, you are sharing!"