One might ask why the Torah chose to use the word “zayit” (olive) in its singular form, rather than “zeitim” , in its plural form.
When any food as small as an egg becomes tamei, it can contaminate other liquids. In smaller amounts though, even if the food becomes impure, it can’t contaminate anything else.
The oil in the olive is enclosed in the physical casing of the olive. As a result, the oil of an olive preserves its purity even after it turns tamei.
For the purpose of transferring tumah, the oil is measured along with the olive even though it is considered distinct within the olive. As a result, the moment the first drop of pressed oil meets the outside skin of a tamei olive larger than an egg, it turns into tamei.
Moshe faced some difficulties when he commanded Bnei Yisroel to contribute olives so that pure oil could be made for the menorah. The olives in their possession were from Mitzrayim. The Jews had second thoughts about using them for the Mishkan since they questioned the olives’ purity.
Moshe recommended they use standard-sized olives, which are roughly equivalent to half an egg. Also, he warned them against squeezing more than one olive at once so that a polluted olive would be insufficient to transfer tumah to the released oil.
By: Tova Bossewitch