Explain Blockchain and bitcoins Like I'm Five

Imagine if you had a key, and a pile of lockable boxes that only your key will open. Anybody can grab a box + leave a message for you in that box. If you're not in the room, somebody who is in the room will take the box to you. The person who brings you the message can't read it because they don't have your key to open the box. You sometimes bring boxes to other folks too, as you move around different rooms.

A blockchain is the rooms and the people. The boxes + keys are the encryption method (typically asymmetric hashing) that keeps the messages in the blockchain safe.

For Bitcoin, imagine that the boxes are really hard to close, and that it costs money to get boxes. The only people who get to make brand new boxes (which they can then sell) are the people who put forth the effort to close the existing boxes. The messages inside the boxes are all records of financial transactions.

Blockchain is a collection of records of things that happened.
It can be used for registering many things.
Bitcoin's blockchain, for example, is a collections of transactions that happened (eg: bob pays alice $10)
It has mechanisms to prevent data tampering, and this allows anyone to validate the records in the blockchain.

Blockchain: Blockchain is a network of blocks (nodes) connected in a chained manner. Generally, blockchains are used to handle the data in a very secure way. The security is obtained by some cryptographic algorithms which are used in each block. For example, in a sequential row of nodes A B C and D. If A wants to send some data to node D which is 2 nodes away. The data will get encrypted and send to the Next node which is available nearer to node A. Here, in this case, it is node B. So the data get passed to node B, then Node B will look for the nearer node (Hear it is C). Then finally it will reach node D. Now you might have got a question like, what if the node B opens and reads the Data? This problem is handled with cryptographic key concepts (i.e) Only the person with the proper key can read the encrypted data. You can search for private and public key concepts if you want more details on that.

BitCoins: Bitcoins are nothing but a virtual money. It is similar to the currencies that you are using every day with some advantages and disadvantages.

Let's say that you would like to make a sum of four unique numbers from 1 to 5 (because 5 year olds might not do well in multiplications). When you sum 1 + 2 + 3 + 4, the result would be 10. But, if you change one of those numbers to 5, the sum of the numbers wouldn't be the same as before.

The concept behind Blockchain is related to something that people called "data integrity", which means every input that is entered to Blockchain depends on previous inputs. For example, in order to get 10 from 1 + 2 + 3 we need to add 4 to those sums.

(considering that 5 year olds might not be able to distinguish between data, information, and knowledge; as well as verification)

Why is Blockchain needed? Blockchain is used to secure (which is, to ensure that something is safe) something that we called "digital transactions/contracts". If you exchange money for your (favourite) foods, you are actually doing a transaction with the person who sells them.

(I doubt that many five-years ever owns money, but perhaps fifth-graders do)

Finally, we call it digital since this one is instead done through computers.