Chai and the number 18
The Hebrew word chai means 'life' in English. Judaism places great value on life and the word 'chai' has become symbolic of that value and the hope that goes with it.
The Hebrew word 'chai' is spelt with the Hebrew letters chet and yud. These letters are often used to form a decorative item, for example to be worn on a chain around the neck.
In Hebrew, each letter has a numeric value. The numeric value of chet is 8, and the numeric value of yud is 10; thus the numeric value of the world chai is 18. Because of this association between chai ('life') and the number, 18 has come to be seen as something of a lucky number. A common practice is for a donation to charity to be made in a multiple of $18 - for example the donation will be $18, or $36, or $90, or $180 (of course, larger multiples are also possible and sometimes given).
When Jews drink alcohol, they often make the toast 'L'chayim!' which means 'to life!'