Last week the question was: Why does this young tomato stem change colors an inch up from the roots?
This tomato plant was grafted, so what you see here is the graft union. The red looking tissue is the scion and the green tissue is the root stock. Grafting is done for a number of reasons, but most commonly on tomatoes for disease resistance. Here is a great publication about grafting for disease resistance.
Here is this week’s question: What is this insect on this sweet potato leaf?