Basar she’nisalem min ha’ayin refers to meat left unsupervised, which leads to a concern that the kosher meat was
switched for meat that is not kosher. Even in the context of one’s backyard, where there is essentially no likelihood that a non-Jew may have eaten the meat and replaced it with non-kosher meat, Rav (Chulin 95a) contends that
we nevertheless must be concerned that an animal picked up the unattended meat and replaced it with non-kosher
meat. This seemingly far-fetched concern, the Yerushalmi (Shekalim 10:2) explains, stemmed from Rav’s observa-
tion that people were being more lax with the kashrus of meat than they should be. He concluded that this measure was therefore in order. The Rishonim dispute whether the halacha is like Rav or like Levi who disagrees with him. Rama (YD 63:2) ruling like Levi, writes that, provided the meat is in the same place one left it, one can assume that it is the original meat. In our case, provided the hamburgers are still on the grill, one can assume nothing has changed. Ashkenazim follow this ruling.

I am preparing a picnic. Can I buy sliced watermelon from the supermarket?

The Shach (YD 96:3) cites a concern that a knife often contains a fatty residue even after it has been washed or wiped with a rag. Therefore, if a non-kosher knife was used to cut kosher food, some of the residue on the knife would transfer to the food. Rama (96:1, 4) writes that with regards to fruit, we can assume that the manufacturer or processor has dedicated utensils. Even if the knife is not dedicated to cutting fruit, however, if large quantities of fruit are being cut or sliced, we can assume that whatever non-kosher residue was on the knife was removed when cut- ting the first few fruit, which are batel (nullified) in the majority of other fruit.