It‘s funny. In Germany a handshake agreement between individuals is worth nothing but between businesses it‘s enforcable like a written contract.
In the US, the general law is that verbal contracts are worth the paper they are written on, which is to say, not at all unless other facts/circumstances support the existence of the verbal agreement and its specific terms. Otherwise anyone could claim to have a contract with anyone else.
For example, I claim to have had a verbal agreement with Mark Z for the development of a social networking site owned by me but built by him. In the absence of any other facts, no court would agree that such a contract exists. However, maybe I show copies of emails where we discussed the idea for a social networking site before it launched. That's some evidence that a verbal agreement might have existed, but not as to what the terms were. Most likely, the parties would settle without going to trial, simply because Mark wants to proceed with the IPO without the sword of litigation hanging over his head--even though he would very likely win. The marginal cost of winning at trial is not worth the much larger cost of the harm to the IPO. (This is very loosely based on the Winklevoss saga.)
Specific types of transaction and certain value thresholds require memorializing.
Facts and circumstances are required to support verbal contracts, but in general verbal contracts very much are enforceable.
There's a reason I brought up the falling apple example. Verbal contracts are enforceable--if they satisfy the same requirements as written contracts. On top of that, the terms of a verbal agreement must be corroborated by other evidence outside of the verbal agreement itself in order to survive litigation--and the trend in the US legal system is to require more supporting evidence.