Designing Databases when relationships change

Newbie question here...
How do I make a good database design when sometimes I don't know all the relationships in advance?

Easy Example 1 - "John Doe's Pizza Shop"
- A "Customer" table and a "Receipt" table. Each customer can have multiple receipts but not the other way around. A receipt is only created when a customer orders. Plain and simple.

Ambiguous Example 2 - "John Doe's Archaeological Research Center"
- A "Tribe" table and a "Tools" table. Each Tool that was excavated could have been used by different tribes. Some tools were only used by one Tribe and some Tools by many Tribes and some Tools will be left without a Tribe name associated because we don't know yet which Tribes used them. At any point in time that new information is discovered, these relationships may change.

Q1. How would I design Example 2?
Q2. Is there a name for these types of designs (where relationships change)? So I can read a good book about it!

Thank you