Grandma,

Bitcoin is like gold.

Except that it is a VERY big complicated numeric word that cannot be imitated.

So, there will only be a certain amount of bitcoins. They wont be able to discover new bitcoins like how they can discover gold deposits.

You can put it in your bag like gold, divide it to small pieces like gold and spend it.

You lose your bitcoins if your wallet gets stolen, just like you would lose your gold if your gold bag was stolen.

You cant send gold over internet, but you can send bitcoins to anyone on internet, instantly.