Let me show you a magic trick for debit/credit cards

Let me show you a magic trick debit/credit cards:

Take out your debit/credit card, and see the digits (For example 𝟰𝟬𝟬𝟬 𝟬𝟬𝟭𝟮 𝟯𝟰𝟱𝟲 𝟳𝟴𝟵𝟵)

Start from the first digit, multiply it by 2 and repeat this process for all digits which are at odd positions. So, after calculation, you would be seeing them like (𝟴-0-𝟬-0 𝟬-0-𝟮-2 𝟲-4-𝟭𝟬-6 𝟭𝟰-8-𝟭𝟴-9)

By this time, you might have noticed that some numbers after multiplication resulted in a 2-digit number. So, for such 2-digits numbers, add both of their digits to generate a single-digit number.
After this stage, your numbers would be like (8-0-0-0 0-0-2-2 6-4-𝟭-6 𝟱-8-𝟵-9)

Now, you have to just add all these digits together, the sum for my example would be: 8+0+0+0+0+0+2+2+6+4+1+6+5+8+9+9=60

This resultant sum would be always divisible by 10 if your debit/credit card number is valid.

This is not a magic trick, but Luhn's Algorithm/Checksum, which is used to validate your input number by card-issuing authorities.

Luhn Algorithm makes it possible to check numbers (Credit card, debit card, SIRET, etc.) thanks to its control key. If a character is misread or badly written, then Luhn's algorithm will detect this error.
Luhn Algorithm is known because MasterCard, American Express (AMEX), Visa, RuPay card and all other credit/debit cards use it.

The algorithm is in the public domain and is in wide use today. It is not intended to be a cryptographically secure hash function; it was designed to protect against accidental errors, not malicious attacks. Most credit cards and many government identification numbers use the algorithm as a simple method of distinguishing valid numbers from mistyped or otherwise incorrect numbers.

Now, let's come to its weakness:
The verification algorithm does not allow the detection of certain permutations of digits. This is the case for pairs 09 and 90: any number containing a 0 replaced by a 9 and a 9 replaced by a 0 has an identical checksum.
Another weakness is the failure to detect a double error like 22 from/to 55 or 33 from/to 66 or even 44 from/to 77.
Also, The presence or absence of leading zeroes at the beginning of the number does not modify the checksum. It can be an advantage as well as a disadvantage.

Tell me in the comments about this magic trick!
(Also, the below image is of a dummy card 😉)