Use saltwater for apples, and keep the pit in the avocado

For apples, skip the lemon juice and give slices a quick saltwater soak; for avocado, keep the pit in and hit the flesh with citrus or vinegar.

The saltwater trick is the lunch-box winner. Dissolve half a teaspoon of salt in two cups of water, soak the apple slices for five minutes, then drain them. They won’t turn brown for hours, and they stay crisp instead of getting mushy like they do with lemon. Your kid won’t taste the salt either — you don’t even need to rinse, but you can if you’re worried.

Avocado needs a different approach. The pit is your friend, but it only protects the spot under it. Brush the cut surfaces with lime, lemon, or even white vinegar, and pack it cut side down in an airtight container. If you’ve got a red onion slice handy, tuck it in next to the avocado — the sulfur compounds slow down browning surprisingly well.

The science is simple: browning needs oxygen plus an enzyme, and salt and acid both mess with that enzyme’s work.

Lunch doesn’t need to look like a science experiment. But it’s nice when it doesn’t look like a crime scene, either.