#include<stdio.h>
#define MAX (100+10)
int ad[26][26]; //인접행렬
int indegree[26], outdegree[26];
int visited[MAX];
int stack[MAX];
int sp = -1;
int N;
char word[MAX][10 + 5];
char *string[MAX];
int rp;
void push(int val){ //나중에 역순으로
    stack[++sp] = val; //출력해야하기 때문에 스택으로 구현
}                       //했습니다.
int pop(void){
    if (sp == -1) return 0;
    return stack[sp--];
}

void makeAd(void){ //인접행렬을 만드는 부분입니다
    int i, j;
    int fir, sec;

    for (i = 0; i < N; i++){
        fir = word[i][0] - 'a';
        for (j = 1; word[i][j] != '\0'; j++);
        sec = word[i][j - 1] - 'a';
        ad[fir][sec]++;
        indegree[sec]++;
        outdegree[fir]++;
    }
}
int checkEuler(void){  //들어오고 나오는 개수를 이용하여
    //int plus=0, minus=0; //서킷,트레일이 아닌것을 거르는부분
    int i;              //입니다.
    int temp;
    int plus = 0, minus = 0;
    for (i = 0; i < 26; i++){
        temp = outdegree[i] - indegree[i];
        if (temp > 1 || temp < -1) return 0;
        if (temp == 1) {
            plus++;
        }
        if (temp == -1) minus++;
    }
    return ((plus == 1 && minus == 1) || (plus == 0 && minus == 0));

}

void DFS(int vertex){
    int i;
    //printf("run DFS()\n");
    for (i = 0; i < 26; i++){
        while (ad[vertex][i]>0){
            ad[vertex][i]--;
            DFS(i);
        }
    }
    push(vertex);
}
void getCircuit(void){   //서킷일 경우와 트레일일 경우를 나눕
    int temp;               //니다.
    int i, j;

    for (i = 0; i < 26; i++){
        if (outdegree[i] == indegree[i] + 1){
            DFS(i);
            return;
        }
    }
    for (i = 0; i < 26; i++){
        if (outdegree[i]){
            DFS(i);
            return;
        }
    }
}

void input(void){
    int i;
    scanf("%d", &N);
    for (i = 0; i < N; i++){
        scanf("%s", word[i]);
    }
}
void printWord(void){
    int i, j, last;
    int fir, sec;
    if (sp != N){
        string[rp++] = "IMPOSSIBLE";
        return;
    }
    fir = sec = pop();
    //printf("fir:%c  sec: %c\n", fir + 'a', sec + 'a');
    for (i = 0; i < N; i++){
        fir = sec;
        sec = pop();
        //printf("fir:%c  sec: %c\n", fir + 'a', sec + 'a'); 
        for (j = 0; j < N; j++){
            for (last = 0; word[j][last] != '\0'; last++);
            if (fir == word[j][0] - 'a' && sec == word[j][last - 1] - 'a' && !visited[j]){
                visited[j]++;
                string[rp++] = word[j];
            }
        }
    }
    //printf("\n");
}
void init(void){
    int i, j;

    rp = 0;

    for (i = 0; i < 26; i++){
        for (j = 0; j < 26; j++){
            ad[i][j] = 0;
        }
    }
    for (i = 0; i < 26; i++){
        indegree[i] = 0;
        outdegree[i] = 0;
    }
    for (i = 0; i < MAX; i++){
        visited[i] = 0;
        string[i] = NULL;
    }
    sp = -1;

}
int main(void){

    int C, i,j;

    scanf("%d", &C);
    for (i = 0; i < C; i++){
        input();
        makeAd();
        if (!checkEuler()) {
            string[rp++] = "IMPOSSIBLE";
        }
        if (rp == 0){
            getCircuit();
            printWord();
        }
        for (j = 0; j < rp; j++){
            printf("%s ", string[j]);
        }
        printf("\n");
        init();
    }

    return 0;
}