A parallel circuit gives the charge more than one route

1. The branches split apart at one junction and rejoin at another, so each coulomb takes one route or the other but never both.
2. Because there are several routes, the current from the source splits between the branches.
3. That makes the current from the source larger than the current in any one branch, and equal to the branch currents added together.
4. Every branch is joined straight across the supply, so every branch has the same p.d. across it.
5. Adding another branch draws more current from the source without changing the p.d. across the branches that were already there.

Adding a parallel branch lowers the total resistance

1. The combined resistance of two resistors in parallel is less than either resistor on its own.
2. That sounds wrong until you think in terms of routes: an extra branch gives the charge an extra way through, so more current flows in total for the same p.d.
3. More current for the same p.d. means less resistance, which follows straight from $R=\frac{V}{I}$.
4. Two identical resistors in parallel give exactly half the resistance of one of them.
5. Each new branch keeps pulling the total down and the current up, which is why loading one extension lead with several heaters can draw a dangerous current.

Lighting circuits are wired in parallel for three reasons

1. Each lamp gets the full supply p.d., so every lamp runs at its proper brightness instead of sharing the supply.
2. If one lamp fails the others stay lit, because the remaining branches are still complete loops.
3. Each lamp can have its own switch in its own branch, so rooms are controlled independently.
4. A series string fails on all three counts, which is why older fairy lights all went out when a single bulb blew.
5. The trade-off is that parallel lamps together draw a larger total current from the supply than the same lamps in series would.

Building the two kinds of circuit on the bench

1. Start at the cell and work round in one direction, adding one component at a time so you do not lose track of the loop.
2. For a series circuit make one continuous loop with no branches, keeping the ammeter in the loop itself.
3. For a parallel circuit connect both ends of each branch to the same two junction points on the main loop.
4. Leave a switch in the main loop so that the circuit can be broken while it is being changed.
5. Check the loop is complete before switching on, then use an ammeter to confirm that the current stays the same or splits as you expected.