Accessing interface in a derived class through the parent class

I'll start off by saying that by no means am I attached to this method, so if anyone has a better idea how to do what I'm trying to do, I would be happy to hear it.

I have an abstract class Thing. From it, I have several derived classes, let's say Car and Box. Both Car, and Box use the same interface, IHealth, which has a property called HP.

In another method, I want a parameter that accepts all classes that derive from Thing and implement IHealth.

 public interface IHealth
 {
       int hp {get; set;}
 }
 
 public abstract class Thing
 {
      //stuff goes here
 }
 
 public abstract class Vehicle : Thing, IHealth
 {
      public abstract int hp {get; set;}
 }
 
 public class Car : Vehicle
 {
      public override int hp;
 }
 
 public class Box : Thing, IHealth
 {
       public int hp;
 }
 
 
 
 
 public class SomeClass
 {
      void FirstMethod()
      {
            //This method gets a Thing and passes it to the next method
            SecondMethod( Thing aThing );
      }
 
      void SecondMethod( Thing aThing )
      {
            // This method receives the Thing and needs to check if the Thing is implementing 
            // the interface, and either return, or retrieve the interface property hp
 
      }
 }
 

I have tried using a generic method, but I don't think that would work since the type that is passed as a parameter must directly inherit the interface (so I wouldn't be able to pass Thing.