Question

bridge, boat
1. People can go across a river on this
(Is 'this' a bridge or a boat?)
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bridge, boat
2. you can go across water in this
(Is 'this' a bridge or a boat?)

Answer 1

1. bridge

2. boat

Answer 2

In everyday English, one may ride either "in" or "on" a boat. One may only cross the water "on" a bridge, though. So #1 can be either a boat or a bridge. #2 only "in" a boat. The shape of a boat allows one to ride "in" or "on" it. A bridge does not enclose one as the hull of a boat does, so only "on" a bridge.