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One Way
Hash functions are one-way functions. Assuming the hash function is a good one, it should be impossible to obtain the input given only the output. The only way to find a message that produces a given hash is to attempt a brute-force search of all possible inputs to see if they produce a matching hash.
A critical operation in most hash functions is the modulo operation (%).
Modulo = Remainder After Division
16 % 7 = 2
Notice how the modulo operation is irreversible. If I only give you the output of x % 7, (which, when x=16 produces 2) you can't know that x was 16. It could have been 9, 23, 30, etc.